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Purpose of Review In this scoping review, we aimed to (1) provide an overview of chronic sleep reduction by systematically reviewing the existing literature (limited to systematic reviews and meta-analyses), (2) investigate the evidence of the effects of short sleep duration on daytime functioning, and (3) identify research gaps in this field. Recent Findings The results showed that (1) clear definitions of chronicity of sleep reduction are lacking—none of the included reviews/meta-analyses provided a full definition—and (2) short sleep duration appeared to be related to obesity and diabetes, whereas relations with cardiovascular disease (CVD), cognitive functioning, emotional problems, general health, and mortality, showed either small effects and appeared to be complex (e.g., for CVD and cognitive functioning), or studies were scarce or completely lacking (e.g., mortality in children/adolescents and emotional problems in adults). Summary Although short or insufficient sleep is highly prevalent and is associated with impaired mental and physical wellbeing, as well as pervasive negative consequences for daytime functioning, the concepts of “chronicity” and “chronic sleep reduction” have not been clearly defined and its effects on health are therefore still largely unknown. Moreover, there are large research gaps concerning studies on the relations between short sleep and health consequences. Further studies are recommended to define and operationalize chronicity of sleep reduction and develop measurements that adequately represent the complexity of the concept. . . . Keywords Sleep reduction Sleep deprivation Sleep problems Daytime consequences Introduction Insufficient and/or inadequate sleep is highly prevalent and is associated with impaired emotional wellbeing [1, 2], daytime E. J. De Bruin and J. F. Dewald-Kaufmann contributed equally to this functioning , and physical health . Even the DSM-5 work. states that (chronic) insomnia does not solely refer to problems This article is part of the Topical Collection on Sleep and Aging with initiating and/or maintaining sleep three times/week for at least 3 months, but also includes impairments in daytime * E. J. De Bruin functioning . Sleep deprivation in the laboratory, in which email@example.com individuals are prohibited to sleep for a defined number of hours and/or nights, is a useful experimental paradigm to Centre for eHealth and Wellbeing Research, Department of study the effects of acute sleep loss under controlled condi- Psychology, Health & Technology, University of Twente, tions. However, it is less representative of individuals’ daily Enschede, The Netherlands life (i.e., ecological validity), in which reduced sleep is usually Research Institute of Child Development and Education, Faculty of experienced over a longer time period (e.g., due to shift work, Social and Behavioral Sciences, University of Amsterdam, lifestyle, work load). Furthermore, other influential sleep var- Amsterdam, The Netherlands iables, such as sleep quality and sleep variability, can also best Hochschule Fresenius, University of Applied Sciences, be studied in somebody’s natural environment. Partial sleep Munich, Germany deprivation can result from fragmented sleep (e.g., in sleep Department of Psychiatry and Psychotherapy, University Hospital apnea), loss of specific physiological sleep stages, or sleep LMU, Munich, Germany restriction, which is characterized by the reduced opportunity 130 Curr Sleep Medicine Rep (2021) 7:129–154 to sleep (e.g., 5, 6, or 7 h/24 h) (e.g., ). The latter is often overview of chronic sleep reduction by its daytime and health referred to as chronic sleep reduction, meaning that sleep is consequences, and by sleep duration and quality, and to iden- reduced over a longer time, such as several days or even tify important research gaps. weeks  although no clear definition of this concept exists. In addition to the problem of ecological validity of labora- tory research, the comparison of the results from epidemio- Methods logical studies is often challenged by the use of different def- initions for sleep and sleep problems (e.g., sleep reduction, We conducted a systematic literature search (PsycINFO, sleep deprivation, sleep problems including sleep disorders, Medline, Scopus, Cochrane library) using variations and com- sleep quality, symptoms of sleep disorders, sleep curtailment, binations of the following keywords dependent on the require- short sleep), and by assumptions about the direction of the ments of the databases that were searched: sleep deprivation, risk, even though most studies concern cross-sectional data insufficient sleep, sleep disruption, systematic review, meta- that are not suited for causal inference. Moreover, many of analysis, suicide, emotions, affect, problem behavior, cog- these definitions contain subjective aspects, such as sleep nitive functioning, memory, executive functioning (see quality and sleepiness. This subjective aspect is also represent- the addendum for a full search report). ed in the classification systems of some sleep disorders (e.g., Articles were included in the paper based on the following insomnia disorder), but is usually very difficult to quantify or inclusion criteria: (1) the paper was published in a peer- even to define clearly and unidimensional. reviewed journal and written in English; (2) the paper was a In the general population, sleep durations seem to decrease systematic review or meta-analysis of original studies; (3) the over the years (e.g., [7, 8]) and many studies have addressed concept of chronic sleep reduction was conceptualized, e.g., a the question of the effects of chronically reduced sleep on definition of too short or insufficient sleep in quantitative daytime functioning, both in longitudinal and cross-sectional terms was given or the hours of sleep, defined as insufficient studies. The “Joint Consensus of the American Academy of sleep in the original studies, were provided; (4) the article Sleep Medicine and Sleep Research Society” concluded that addressed a healthy population. Articles were excluded (1) if healthy adults should sleep on average 7 or more hours per they were not written in English or not published in a peer- night, as less sleep is related to negative health outcomes (e.g., reviewed journal; (2) if they were no systematic review/meta- weight gain, obesity, diabetes, hypertension, heart disease, analysis but an original study (e.g., experimental study, cross- stroke, depression, impaired immune function, increased pain, sectional study); (3) if sleep problems referred to sleep disor- increased risk of accidents, increased risk of death) . ders (e.g., insomnia, sleep apnea) or addressed a specific pop- However, the concept of chronicity still remains vaguely de- ulation that is assumed to experience insufficient sleep be- fined or even undefined, meaning that little is known yet, of cause of specific circumstances (e.g., pregnant women); (4) how many nights sleep has to be reduced by how many hours, if the review focused on participants with a certain disorder, and in which way (e.g., variability, circadian shift) in the con- such as psychiatric disorders (e.g., bipolar disorders), demen- text of personal sleep need and satisfaction (e.g., ), to tia, Parkinson’s disease, chronic pain, physical illness (e.g., negatively affect somebody’s daytime functioning. This issue, cancer, epilepsy); (5) if the review/meta-analysis included on- together with problems of ecological validity and definitions ly experimental studies in the laboratory for short-term sleep for sleep and sleep problems, leads to the question whether deprivation or extension. The following characteristics were chronic sleep reduction can be defined in such a way that it coded: (1) goals/aims, (2) number and nature of included stud- catches both the quantitative aspect from laboratory studies ies, (3) participants’ age, (4) operationalization/definition of and the broader phenomenological descriptions and defini- chronic sleep reduction (including definition of sleep charac- tions from epidemiological or clinical studies. teristic, definition of short sleep duration), (5) assessment In comparison to systematic reviews, scoping reviews aim methods of chronic sleep reduction (e.g., questionnaire, to map the existing literature, to identify research gaps, to give polysomnography), (6) consequences of chronic sleep reduc- a broader overview of the literature on a specific topic, and to tion (e.g., effects on daytime functioning, mental health, phys- provide ideas for future research (e.g., ). Concerning the ical health), (7) quality assessment of the included studies. topic of this review, two recent studies aimed to summarize A total of 1726 unique articles were identified with the the current knowledge concerning the relationship between initial search that was conducted on March 6, 2017. After sleep, developmental aspects, and health in children and ado- screening titles and abstracts, 1684 articles were excluded, lescents [12� , 13]. Although these studies give a good over- resulting in 42 articles, which were fully read and coded by view of the research field, both were limited to children and two independent researchers (EdB, JDK). From these 42 arti- adolescents, and one focused on research of the last 5 years. In cles, 27 did not fully meet the in- and exclusion criteria and order to summarize the existing literature in this field, we, were excluded, resulting in 15 articles. On June 18, 2018, we therefore, conducted a scoping review, aiming to provide an updated the search and identified 370 additional unique Curr Sleep Medicine Rep (2021) 7:129–154 131 articles. After screening, 19 of these articles were fully read, the applied search strategy. Because different reviews/ coded, and discussed, resulting in five additional articles. We meta-analyses about similar outcomes often included repeated this search strategy on May 7, 2019, identified several of the same original articles, we calculated and 297 additional unique articles from which 16 were fully reported the unique number of articles and participants read, coded, and discussed, and two additional articles (i.e., deleted the overlap). The numbers of articles and were included. This brought the total number of includ- participants in Table 1, however, are as the authors ed articles in the present study to 22. Figure 1 presents originally reported them in the reviews/meta-analyses. Fig. 1 Database search and article Search with variations of keywords, dependent on the requirements of the databases: selection sleep deprivation, insufficient sleep, sleep disruption, systematic review, meta-analysis, suicide, emotions, affect, problem behavior, cognitive functioning, memory, executive functioning, health (see the addendum for a full search report). Medline: PsychINFO: Scopus: Cochrane: 1.497 Articles 749 Articles 832 Articles 120 Articles Total: 3.198 Articles Excluded 1.472 duplicate articles Title and abstract screening: did not meet Result: 1.726 unique articles inclusion criteria: 1.684 Articles Full text screening: 27 Articles Result: 42 Articles did not meet inclusion criteria Result: 15 Articles Second search: 370 additional unique articles Title and abstract screening: 351 articles did not meet inclusion criteria Full text screening of 19 articles: 14 did not meet inclusion criteria Result: 20 Articles 5 additional articles included Third search: 297 additional unique articles Title and abstract screening: 282 articles did not meet inclusion criteria: Full text screening of 15 articles: 13 did not meet inclusion criteria Included: 22 Articles 2 additional articles included 132 Curr Sleep Medicine Rep (2021) 7:129–154 Table 1 Characteristics of the included reviews and meta-analyses Reference Aims Included studies and design Definition insufficient Assessment Consequences of Quality assessment sleep insufficient sleep Anothaisintawee Systematic review and 36 cohort studies (1,061,555 participants in total, Relative risks for Self-report Pooled RR for diabetes: Assessment: Risk of et al., 2016 meta-analysis of the 583,263 participants on sleep duration) self-reported diabetes Short sleep 5 h/day: bias was assessed (Thailand) relative risk of sleep Age: adults were estimated for 1.48 (95% CI = with the modified (disturbances) for Sleep duration: 14 studies self-reported short (= 1.25–1.76) Newcastle-Ottawa diabetes, compared Sleep quality: 11 studies 5-=6 h/day), normal Short sleep 6 h/day: scale (NOS) to overweight, Obstructive sleep apnea (OSA): 8 studies (7–8h/day), andlong 1.18 (95% CI Results: In all studies, family, physical ac- Shift work: 10 studies (≥ 9 h/day) sleep, and =1.10–1.26) there was a low risk tivity. for sleep quality (de- Long sleep 9 h/day: of bias concerning fined as presence or 1.36 (95% CI = adjustment for absence of an insom- 1.12–1.65) confounders. Low nia symptom - diffi- Sleep quality (insomnia risk of bias ranged culty initiating or symptoms): 1.40 from 30.5% maintaining sleep), (95% CI = (ascertainment of obstructive sleep ap- 1.21–1.63) exposure) to 100% nea (OSA), or shift (Insomnia symptoms, (adjustment of work. obstructive sleep confounding factors). Definition of chronicity apnea (OSA), and not reported. shift work are outside the scope of the present study) Overall conclusion: Significant small effect of sleep duration and sleep quality for diabetes comparable to traditional risk factors such as physical inactivity. Aziz et al., 2017 Systematic review of Total 31 (54,120 participants) Subjective sleep quality Polysomnography, Association between Quality assessment of (USA) how sleep duration Age: adults is defined as “one’s actigraphy, subjective short sleep studies not reported. and sleep quality 28 cross-sectional perception that they self-report duration and coronary artery affect the burden of 3 cohort fall asleep easily, get subclinical sufficient duration so calcium (CAC) and cardiovascular as to wake up feeling carotid intima-media disease. rested, and can make thickness (CIMT) it through their day was present, which without experiencing was less consistent excessive daytime with endothelial dys- sleepiness.” function (ED) and Measured by arterial stiffness. self-report such as Objective short sleep Pittsburg Sleep duration: There was Quality Index. an association with Curr Sleep Medicine Rep (2021) 7:129–154 133 Table 1 (continued) Reference Aims Included studies and design Definition insufficient Assessment Consequences of Quality assessment sleep insufficient sleep Objective sleep quality CIMT, which was is defined by inconsistent sufficient duration (7 concerning CAC. h), high efficiency There was no (85%), low association with ED. fragmentation ( 25), Poor subjective sleep and proper sleep quality: There was an staging (1–4and association between REM), measured by ED and arterial actigraphy and stiffness, which was polysomnography. inconsistent Definition of chronicity concerning CAC and not reported, but CIMT. implicit 1 month in Objective sleep quality: measurements such There was an as Pittsburg Sleep association between Quality Index. CIMT and ED, which was inconsistent concerning CAC. Overall conclusion: There appeared to be a relationship between short sleep duration and poor sleep quality. Brewster et al., Systematic review of Total 29 studies (80,176 participants) Mixed definitions for Self-developed Sleep duration: as a Quality assessment of 2015 (USA) the relationship Age: 60 years sleep duration: e.g., questionnaires, or continuous variable studies not reported. between sleep 19 cross-sectional Short sleep duration as proxy measures of mixed results, but parameters and 2 prospective 5h,6h, 7h sleep, sleep diaries, overall no relation cognition in older 8 longitudinal Long sleep duration as some actigraphy with global adults without sleep (Some of those combined) 8h,8.5 h, 9 h cognition. When disorders. No definition of cut-offs dichotomized, a for sleep parameters: U-shaped relation Sleep latency emerged. Wake after sleep onset Sleep complaints: Sleep efficiency mixed results imply a General sleep relation between complaints sleep problems and (operationalized as, impaired cognition. e.g., problems falling Sleep efficiency: or staying asleep, relations between Pittsburg Sleep objective sleep Quality Index score efficiency and 134 Curr Sleep Medicine Rep (2021) 7:129–154 Table 1 (continued) Reference Aims Included studies and design Definition insufficient Assessment Consequences of Quality assessment sleep insufficient sleep 5, global sleep cognition, but not quality, total wake with subjective sleep time, sleep efficiency. fragmentation). Sleep onset latency: Definition of chronicity mixed results, most not reported, but consistent relation implicit 1 month in with global measurements such cognition. as Pittsburg Sleep Wake after sleep onset: Quality Index. mixed results, some indications of relations with specific cognitive domains (executive function, attention, episodic memory, verbal fluency) and global cognition. Overall conclusion: Inconsistent relations between short sleep duration, sleep complaints, and sleep problems with cognition in older adults exist. Depressive symptoms, undiagnosed (other) sleep disorders, and medical conditions may modify these re- lations. Cappuccio et al., Meta-analysis and Total 45 (45,264 children, 791,943 adults) Short sleep duration Use of self-report, Short sleep duration Quality assessment of 2008 (UK) review of the Age: 2–102 years defined as: objective measures, pooled OR for studies not reported relationship between Included in meta-analysis of cross-sectional associa- For children: 10 h or 10 and sleep diaries. obesity: short sleep duration tions h/night. Children: 1.89 (95% CI and obesity at 12 children (30,002) Foradults:5hor5 =1.49–2.43) different ages, and an 18 adults (604,509) h/night Adults: 1.55 (95% CI = estimate of the risk. Reviewed Chronicity not defined. 1.43–1.68) 7 children (15,262) The pooled β for short 8 adults (189,927) sleep duration in adults was −0.35 (95% CI = Curr Sleep Medicine Rep (2021) 7:129–154 135 Table 1 (continued) Reference Aims Included studies and design Definition insufficient Assessment Consequences of Quality assessment sleep insufficient sleep −0.57–−0.12) unit change in BMI per hour of sleep change. Overall conclusion: Cross-sectional stud- ies of children and adults with short sleep duration indi- cate an increased risk of obesity. Chaput et al., Systematic review of 141 articles (110 unique samples, 592,215 Mixed definitions of Sleep duration was Overall conclusion: Assessment: Quality of 2016 (Canada) the relationships participants) short or long sleep: measured objectively Shorter sleep the evidence was between sleep Age: children/adolescents (5–17 years) e.g., 10.5 h for (polysomnography duration was assessed with the duration and health Adiposity (n = 71) increased vs. 8.1 h or actigraphy) in 29 associated with Grading of in children and youth Emotion regulation (n = 62) for decreased sleep. studies. impaired physical Recommendations Cognition and academic achievement (n = 6) Short sleep duration as The remaining 157 and mental health in Assessment, Quality of life/wellbeing (n = 3) ≤ 6 h/night. studies used children and Development and Harms/injuries (n = 4) Short sleep as children subjective self-report adolescents (i.e., Evaluation Cardiometabolic biomarkers (n = 19) who slept 10 h/night. or parent-report. excess adiposity (GRADE) frame- Some experimental (i.e., sleep restriction studies) Short sleep as (58/71 studies), work. randomized controlled trial (n = 6), longitudinal adolescents who decreased emotional Results: The quality of studies (n = 33), cross-sectional studies (n = 145), slept 7 h/night regulation (49/62 evidence ranged case-control studies (n = 2) Definition of chronicity studies) and from very low to not reported academic high across study achievement (14/21 designs and health studies), and lower indicators (the quality of possible 4 categories life/wellbeing (3/3 were “high,” studies). “moderate,”“low,” Associations were “very low”). less clear concerning the relationship between sleep duration and cognition, harms/injuries, and cardiometabolic marker (none of the included studies reported a clear positive association). Chaput et al., Systematic review of 69 articles (62 unique samples, 148,524 participants) Mixed definitions of Sleep duration was Overall conclusion: Assessment: Quality of 2017 (Canada) the relationships Age: children (0–4years) short and long sleep assessed by parental Associations were evidence was between sleep Randomized trials (n = 3) duration: e.g., short report in 48 studies, indicated between assessed with the 136 Curr Sleep Medicine Rep (2021) 7:129–154 Table 1 (continued) Reference Aims Included studies and design Definition insufficient Assessment Consequences of Quality assessment sleep insufficient sleep duration and health Non-randomized intervention studies (n = 1) sleep 690 min, long objective measures short sleep duration Grading of indicators in the early Longitudinal studies (n = 16) sleep ≥690 min. 10 h (polysomnography and adiposity (20/31 Recommendations years (0–4 years). Cross-sectional studies (n = 42) vs. 11 h per night. or actigraphy) in 10 studies), poor Assessment, Longitudinal studies that included cross-sectional Short sleep as 8 h/night. studies, and by emotion regulation Development and analyses (n = 7) Definition of chronicity actigraphy and sleep (13/25 studies), Evaluation (GRADE) frame- Adiposity (n = 31), emotion regulation (n = 25), not reported. logs in 11 studies. increased screen time cognitive development (n = 16), motor (5/5 studies), and work. development (n = 2), growth (n = 2), increased risk of Results: The quality of cardiometabolic health (n = 0), sedentary behavior injuries (2/3 studies). evidence ranged (N = 5), physical activity (n = 4) For the associations from very low to with cognitive high across study development, motor designs and health development, indicators. physical activity, and (The possible 4 quality of categories were life/wellbeing, the “high,”“moderate,” evidence was less “low,”“very low.”) clear. Chen et al., 2008 Systematic review and 17 studies (55,791 participants) Standardized reference Sleep duration was General pediatric Quality assessment of (USA) meta-analysis of epi- Age: children/adolescents (3–19 years) for sleep duration assessed by population: OR:1.58 studies not reported demiologic evidence 3 cohort studies was defined as ≥11 h self-report in 15 (95% CI = on the relation be- 12 cross-sectional studies for children aged 5 studies, by self-report 1.26–1.98). No clear tween sleep duration 2 case-control studies years, ≥10 h for and time diaries in 1 dose-response rela- and childhood obesi- children aged study, and by tionship was found. ty. between 5 and 10 actigraphy in 1 study. Comparison of short years, and ≥ 9h for sleep duration vs. children ≥10 years of recommended sleep age. duration: OR = 1.60; Children’ssleep 95% CI = 1.22–2.10 duration was Comparison of shortest contrasted with the sleep duration vs. recommended level recommended sleep of sleep duration duration: OR = 1.92; (e.g., 9hfor children 95% CI = 1.15–3.20 aged ≥10 years). Pooled OR for obesity: Dose-response 0.91 (95% CI = investigated by 3 0.84–1.00) for each levels of short sleep hour increased sleep duration: duration. Shortest, shorter and Overall conclusion: short: Short sleep duration Children 5 years: 9 h, was clearly 9–10 hr, 10–11 hr. associated with an Curr Sleep Medicine Rep (2021) 7:129–154 137 Table 1 (continued) Reference Aims Included studies and design Definition insufficient Assessment Consequences of Quality assessment sleep insufficient sleep Children 5-10 years: 8 increased risk of h, 8–9h,9–10 h. childhood obesity. Children10years:7h, 7–8h,8–9h. Definition of chronicity not reported. Chiu et al., 2018 Systematic review and 10 cross-sectional (598,281 participants), with 1 Categories differed per Self-reported sleep Risk of suicidal ideation Assessment: Study (Taiwan) meta-analysis on the study reporting on 2 samples and 2 studies study. In the majority duration frequency (13 reports): pooled quality was assessed dose-response rela- reporting sex-specific ratios (13 reports in total). of studies, categories question in all but OR .55 (95% CI with the Critical tionship between Age: 14.6–16.3 years were clustered into one study, which =.47–.65) Appraisal Checklist, sleep duration and <4/5,5,6,7,8,9, used the Pittsburg Risk of suicidal plans (6 recommended by the risk of suicidality in >8/10. Most studies Sleep Quality Index. reports): pooled OR Joanna Briggs adolescents. used a reference cat- .50 (95% CI Institute. egorywith6h =.44–.56) Results: All articles met sleep/night. Risk of suicide attempts 6ofthe 8 criteria.No Definition of chronicity (11 reports): pooled article met all not reported. OR: .52 (95% CI = required criteria; .41–.66) study quality was 8–9 h/sleep provides the especially low lowest risk of suicide concerning the attempts. assessment tools for Overall conclusion: sleep and suicidality. Sleep duration was found to be an independent risk factor for the development of suicidality. Shorter sleep durations are associated with a higher risk of suicide plans. There appeared to be a U-shaped association between sleep dura- tion and suicide/- attempts. Fatima et al., Review and 22 studies (47,223 participants) Normal sleep category Sleep duration Association between Assessment: Quality of 2015 meta-analysis of the Age: children/adolescents (0.5–18 years) basedonSleep measured by self- or short sleep duration the studies was (Australia) prospective associa- All longitudinal studies (follow-up ranged from 1–27 Health Foundation parent-report in most and obesity assessed using the tion of short sleep years) Recommendation, studies, 6 studies Pooled OR: 2.15 (95% Grading of duration with obesity 12 studies on children category less than used time diaries or CI = 1.64–2.81) Recommendations in children and ado- 6 studies on children and adolescents this recommendation sleep logs, 2 studies The strongest Assessment, lescents. 4 studies on adolescents defined as short sleep used validated association was Development and 138 Curr Sleep Medicine Rep (2021) 7:129–154 Table 1 (continued) Reference Aims Included studies and design Definition insufficient Assessment Consequences of Quality assessment sleep insufficient sleep 11 studies (13 samples, 24,821 participants) included duration. questionnaires, 2 reported in studies Evaluation in meta-analysis Recommendation studies actigraphy, where reference (GRADE) and the basedon24h and 1 study sleep was equivalent Methodological Short sleep duration polysomnography. to normal sleep Evaluation of varied: duration: OR 2.32 Observational Younger children (95% CI = Research 10–12 h/night 1.77–3.05). (MEVORECH). Older children from A trend appeared of a Results: Quality of 7.5– 10.5 h/night stronger association individual studies Adolescents 6– 11 h/- for adolescents (OR: ranged from 7 to 12 night 2.06; 95% CI = (maximum points: The reference category 1.55–2.74) compared 17) cut-off was 0.5–1.0 h to children (OR: greater than the 1.76N 95% CI = “short” sleep catego- 1.44–2.15). ry cut-off. Overall conclusion: The Definition of chronicity risk for obesity is not reported. higher in children and adolescents with shorter sleep duration compared with those with normal or longer sleep duration. Felső et al., 2017 Systematic review of 33 studies (22,920 participants) Sleep duration Sleep duration Overall conclusion: Quality of studies not (Hungary) the association of Age: children/adolescents (0.5–19 years) reference values for measured objectively Sleep duration assessed. sleep duration and 3 randomized controlled trials normal sleep differed (e.g., actigraphy, appears to influence obesity and a 3 cohort studies between studies, polysomnography). weight gain in discussion of its 26 cross-sectional studies e.g.,: children. Regarding underlying 1 cross-sectional and cohort study Children3years:12h underlying (patho)physiological Some studies included experimental sleep restriction. Children 7 years: 9 h mechanisms that mechanisms 23 studies on obesity Children 8–11 years: 9 explain this relation, 6 studies on diet h, 10 h short sleep duration 6 studies on physical activity Children/adolescents could be linked to the 2 studies on screen time 8–16 years: 6.5 h development of 4 studies on insulin sensitivity4 studies on ghrelin and Children 8–11 years: insulin resistance, leptin levels short: 7.47–8.56 h, sedentarism, and long: 9.32–10.50 h unhealthy dietary Children 9–11 years: 9 patterns, whereas years: 10 h; 10 years: nothing could be said 9.75 h; 11 years: 9.5 about other h mediators (physical Adolescents 14–16 activity, screen time, years: 7 h change in ghrelin, and leptin levels) Curr Sleep Medicine Rep (2021) 7:129–154 139 Table 1 (continued) Reference Aims Included studies and design Definition insufficient Assessment Consequences of Quality assessment sleep insufficient sleep Adolescents 15–19 years: 8 h/night Definition of chronicity not reported. Fobian et al., Systematic review 24 studies (35,039 participants), children/adolescents Sleep duration Sleep duration Overall conclusion: Assessment: Risk of bias was assessed 2018 (USA) assessing sleep, 9 studies on hypertension reference values measured in most Obesity, hypertension, and 15 studies on obesity differed between studies by parent hypertension, and using the RTI Item cardiovascular risk 13 studies on glucose or insulin studies, e.g.,: report, child insulin sensitivity Bank. 8 studies on lipids Children 3–11 years: self-report, showed the strongest Results: Only 1 study 3oninflammation sleep ≤ 9 questionnaires (e.g., evidence of an had low risk of bias, hAdolescents 10–15 Pittsburg Sleep association with 3 had moderate risk years: sleep 8 hSome Quality Index, sleep duration and of bias, all others had studies included BEARS).In4studies some evidence was high risk of bias sleep stages from polysomnography indicated of a link (categories are low, polysomnography in wasused, andin2 between insulin medium, high). analyses. studies actigraphy. resistance and sleep. Definition of chronicity No strong evidence not reported. appeared for an association between glucose or insulin, lipids, or inflammation and sleep duration. Frange et al., Association between 5 studies (159,044 participants) Sleep duration All studies used Overall conclusion: Quality assessment of 2014 (Brazil) sleep duration and Age: adults (17–95 years) reference differed one-item questions Short and long sleep studies not reported. self-rated health. 4 cross-sectional studies between studies. All on sleep duration duration had negative 1 longitudinal (cohort) study studies used such as “On average, impact on SRH, even categories ranging how many hours of when analyses were from very short (e.g., sleep do you get in a adjusted for possible 6h)to verylong 24 hour period?” confounders. (e.g., 10 h). Definition of chronicity not reported. Gallicchio & Systematic review and 23 studies (2,218,284 participants) A single short sleep All studies examined Pooled relative risk Quality assessment of Kalesan, 2009 meta-analysis of the Age: adults (18–102 years) group ( 7 h for most sleep duration using (RR) of short sleep studies not reported (USA) associations between All prospective cohort studies with years of follow-up studies) and long a single self-reported duration: sleep duration and between3.5and25years. sleep group ( 9 h for survey item. All-cause mortality: mortality. most studies) were 1.10 (95%; CI = comprised by 1.06–1.15) pooling sleep Cardiovascular-related duration categories mortality: 1.06 (95% from the individual CI = 0.94–1.18) studies, and compared to a 140 Curr Sleep Medicine Rep (2021) 7:129–154 Table 1 (continued) Reference Aims Included studies and design Definition insufficient Assessment Consequences of Quality assessment sleep insufficient sleep medium sleep Cancer-related duration group mortality: 0.99 (95% (7–7.9 h). CI: 0.88–1.13) Definition of chronicity Overall conclusion: not reported. Short sleep increases the risk for mortality. Itani et al., 2017 Systematic review and 108 studies (5,172,710 participants) Because normal Some studies included Risk ratios of short Assessment: The (Japan) meta-analysis of 153 datasets for nine outcomes, 14 of these datasets reference values for objective measures, sleep duration for: Newcastle-Ottawa dose-response of not included in meta-analyses. sleep duration vary but only subjective Mortality: 1.12 (95% CI scale (NOS) was short sleep duration Age: 20 years at baseline. among cultures and measurements of =1.08–1.16) used to assess the and mortality, health All prospective cohort studies, with follow-up rang- ethnicities, definition sleep duration (e.g., Diabetes mellitus: 1.37 studies’ quality. complications, and ing from 1–30 years of short and normal self-report) were (95% CI = Results: Scores ranged diseases. Some studies used multiple outcomes: sleep duration was included in analyses. 1.22–1.53) from 7to9 (a score 38 studies on mortality basedonthe original Hypertension: 1.17 of 8 was considered 20 studies on diabetes paper, but most (95% CI = as high quality). 11 studies on hypertension studies defined short 1.09–1.26) Most studies were 25 studies on cardiovascular disease sleep duration as less Cardiovascular considered to be 14 studies on stroke than5h or 6 hand diseases: 1.16 (95% high-quality studies. 20 studies on coronary heart disease normal sleep CI = 1.10–1.23) 21 studies on obesity2 studies on dyslipidemia2 duration as 7 h. Coronary heart studies on depression Definition of chronicity diseases: 1.26 (95% not reported. CI = 1.15–1.38) Obesity: 1.38 (95% CI =1.25–1.53) Short sleep duration 6 h was associated with an increase in mortality. Overall conclusion: Short sleep is related to physical health and mortality. Jensen et al., Systematic review of Overall conclusion: Assessment by the 7 studies (8849 participants) Definitions of sleep One study used 2013 the association Age: children/adolescents (7–15 years) duration and sleep actigraphy; all others There is an indication Joanna Briggs (Australia) between poor sleep 5 studies on physical activity quality differed used self-report or that poor sleep Institute quantity/quality, and 5 studies on diet between studies: parent report, includ- quantity/quality may Meta-Analysis of dietary intake, and Children 7–9years: ing insomnia symp- be associated with Statistics Assessment physical activity in sleep duration 9 h, toms. poorer dietary and and Review children and 9–10 h, 10–11 h, 11 physical activity be- Instruments, using 9 adolescents. hChildren9.56years: haviors. questions, of which 2 self-reported average were not applicable sleep latency. because all 7 includ- Adolescents 11–16 ed studies were years assessed by cross-sectional with Curr Sleep Medicine Rep (2021) 7:129–154 141 Table 1 (continued) Reference Aims Included studies and design Definition insufficient Assessment Consequences of Quality assessment sleep insufficient sleep actigraphy: total only one time point sleep time 7.68 h measurement. (5.7–9.1); sleep dis- Results: 2 studies turbance time 1.17 h scored positive on 4 Adolescents 14.6 years: questions, 2 studies Insomnia symptoms scored positive on 3 and bedtime questions, 2 studies reported. scored positive on 2 Definition of chronicity questions, and 1 not reported, but study scored positive implicit in insomnia on only 1, out of 7 symptoms. questions. Improvements were recommended for randomization, use of objective measurement instruments, and sufficient descriptions of groups. Li et al., 2017 Systematic review and 32 studies Sleep duration differed Most parent report or Pooled relative risk Assessment: Quality of (China) meta-analysis of pro- 12 studies included in meta-analysis (44,200 partici- between studies but self report, some time (RR) of short sleep each study was spective cohort stud- pants) were categorized diaries, one duration for obesity: evaluated using the ies on the associa- Age: children/adolescents (0.5–18 years), all cohort from short to long, polysomnography. 1.45 (95% CI = Newcastle-Ottawa 1.14–1.85). Scale (NOS). tions between sleep studies e.g.: duration and obesity Children 0.5 years: 12, Overall conclusion: Results: Scores ranged in children. ≥ 12 h Short sleep is related from 5to8. Children 3 years: 10.5, to obesity in children. goed vind. Ook hier 10.5–10.9, 11–11.9, weer het footnote 12 h weg lijkt me Children 3–12 years: 8, 8–9, 9–10, 10–11, ≥ 11 h Children 0–13 years: ≤ 25th percentile of age-specific sleep recommendation Adolescents 12–18 years: 6, 6–8, 8–11, 11 h Definition of chronicity not reported. 40 studies (2,200,425 participants) 142 Curr Sleep Medicine Rep (2021) 7:129–154 Table 1 (continued) Reference Aims Included studies and design Definition insufficient Assessment Consequences of Quality assessment sleep insufficient sleep Liu et al., 2017 Meta-analysis of sleep 24-h sleep duration: 28 studies Categories differed Mostly self-reported 24-h sleep duration: Quality assessment of (China) duration and Night-sleep duration: 14 studies between studies. 7 h questionnaires and J-shaped association. studies not reported. all-cause mortality Age: 20–97 years was used as structured interviews Every 1-h of sleep reference. increased the risk of Definition of chronicity all-cause mortality not reported. slightly (RR = 1.07; 95% CI = 1.03–1.11) 4 h of sleep: RR = 1.05 (95% CI = 1.02–1.07) 5 h of sleep: RR = 1.06 (95% CI = 1.03–1.09) 6 h of sleep: RR = 1.04 (95% CI = 1.03–1.06) Night-sleep duration: J-shaped association, indicating relation- ship between short sleep and all-cause mortality Overall conclusion: There seems to be no or a small effect of short sleep on all-cause mortality. Lo et al., 2016 Systematic review and 18 studies (97,246 participants) Reference for normal Self-reported sleep Odds for poor cognitive Assessment: The (Singapore) meta-analysis of ef- 35 independent samples sleep duration ranged duration, most with functioning was 1.4 quality of the studies fects of short sleep Age: adults (≥ 55 years) from 5 to 9 h: most one-item questions. times higher among was assessed with the duration on cognitive 11 cross-sectional studies commonly used were short sleepers than modified version of performance in older 7 prospective cohort studies with follow-up between 7h,8h, and 7–8h normal sleepers; the Downs and Black adults 1 and 22 years Short sleep duration: 5 h multiple-domain Quality Index score Long sleep duration: ≥ tasks: OR = 1.28 system 9h (95% CI = Results: All studies Definition of chronicity 1.07–1.53) and OR = were of satisfactory not reported. 1.24 (95% CI = quality. Scores 1.02–1.50) without ranged from13to16 studies on (cross-sectional Alzheimer’s studies) and 13–19 disease/dementia; (prospective studies) executive functions: (maximum scores: OR = 1.33 (95% CI = 16 for 1.11–1.46); verbal cross-sectional; 19 Curr Sleep Medicine Rep (2021) 7:129–154 143 Table 1 (continued) Reference Aims Included studies and design Definition insufficient Assessment Consequences of Quality assessment sleep insufficient sleep memory: OR = 1.33 for prospective stud- (95% CI = ies) 1.19–1.49), working memory capacity: OR = 1.35 (95% CI = 1.15–1.49); speed of processing: OR = 1.36 (95% CI = 0.88–2.12). Overall conclusion: Mixed results with small effect sizes exist for the relationship between short sleep and cognitive functioning; however, short sleep durationisseenasa risk factor for cognitive aging. Magee et al., Systematical review of 20 studies (123,354 adult and 10,959 child Sleep duration All children studies Studies with children Quality assessment of 2012 (USA) the relationship participants) reference differed used parental report consistently reported studies not reported. between sleep Only longitudinal studies included between studies. and one also a relationship duration and Age: adults and children Some adult studies self-report. In adult between short sleep subsequent weight 13 adults (18–81 years at baseline) used categories studies, all used duration and weight gain in observational 7children(0–12 years at baseline) ranging from short self-report, except gain, except for one longitudinal human (e.g., 5–6h,or ≤ 5h) one that used study with older studies. to long (e.g., 9–10 h, actigraphy. children between 8.0 or ≥ 8 h), compared and 12.9 years of to normal (7–8h) age. Studies with adults sleep duration. In children studies cate- showed more gories ranged for variation, with four short sleep duration studies that found an (e.g., 12 h/day for association between babies, or 10.5 h for short sleep duration age 3 years). and weight gain, four Definition of chronicity with an association not reported. with weight gain for both short and long sleep, and the remaining five 144 Curr Sleep Medicine Rep (2021) 7:129–154 Table 1 (continued) Reference Aims Included studies and design Definition insufficient Assessment Consequences of Quality assessment sleep insufficient sleep studies reporting no association. Overall conclusion: Short sleep duration is related to weight gain in children; however, this relationship is not clear in adults. Miller et al., Systematic review and 42 studies (116,523 participants, 63 cohorts) Short sleep duration as Actigraphy was used in Pooled relative risk Assessment: Quality of 2018 (UK) meta-analysis on the 20 studies on obesity defined in included 4 studies; 1 study (RR) overweight or included studies was prospective relation- 14 studies on change in BMI Z-scores articles was used, used obesity in infancy: assessed by the ship between sleep 16 studies on change in BMI differing for age plysomnography; all RR: 1.40 (95% CI = Downs and Black and obesity in a pe- Infants(0to3years) groups, e.g., short others used parent 1.19–1.65) Quality Index score diatric population. Early childhood (3 to 9 years) sleep duration: ≤ report and/or self re- Early childhood: RR: system. Middle childhood (9 to 12 years) 11.25 h, compared to port. 1.57 (95% CI = Results: Scores ranged Adolescents (12 to 18 years) normal sleep: 1.40–1.76) from 12 to 18 11.25 at age 3 or 4 Middle childhood:RR: (maximum score: years, or short sleep (95% CI = 2.23; 20). 7 studies scored duration: 10.5 h 2.18–2.27) 15 and were removed (lowest quarter) Adolescence:RR: 1.30 in sensitivity compared to 12 h (95% CI = analyses, which did (highest quarter) at 1.11–1.53). not impact the results age 3 years. Short Sleep duration was also sleep duration 12 h associated with a compared to ≥12 h significant change in for children from 6 BMI z-score RR; months to 2 years. −0.03 (95% CI = Short sleep duration −0.04 to −0.01) and 11 h and long sleep in BMI RR: −0.03 duration 11.5 h (95% CI = −0.04 to compared to −0.01) 11–11.5 h for 5, 7, 9, Overall conclusion: and 11 years. Short sleep duration Short sleep duration: seems to be a risk ≤8.5h compared to factor concerning the ≥9.5h. Short sleep development of duration: 6 h obesity in infants, compared to 8 h. children, and Sleep duration also adolescents. analyzed as continuous variable. Definition of chronicity not reported. Curr Sleep Medicine Rep (2021) 7:129–154 145 Table 1 (continued) Reference Aims Included studies and design Definition insufficient Assessment Consequences of Quality assessment sleep insufficient sleep Nielsen et al., Systematic review of 7 reviews from before 2008, 15 studies (prospective Sleep duration defined Studies used parent Overall conclusion: Quality assessment of 2011 epidemiological studies and cross-sectional studies) not included in differing between report or personal Short sleep duration studies not reported. (Denmark) studies on the these previous reviews (86,181 child and 13,117 studies. parent interview, and is consistently relation between adult participants) Infants: 12 h of one study used associated with sleep duration and Age: sleep/day compared parental sleep diary. increased risk of obesity. Review of 8children with 12 h day development of reviews on this 7 adults Short sleep duration obesity in children subject from before (10.5 h) at age 2 and young adults, but 2008, and reviews of Definition of chronicity not consistently so in unique new studies not reported. older adults. Causal not included in any interpretation of the of the previous association is reviews. hampered by several epidemiological problems possibly leading to biases as well as by more fundamental conceptual problems. Zhou et al., 2019 Systematic review and 12 prospective cohort studies (16 reports; 198,579 In the majority of Not reported Pooled RR for risk of Assessment: Quality of (China) dose-response meta-- participants) studies, exposure obesity: 1.09 (95% included studies was analysis to investi- Age: Adults ( 18 years at baseline) categories were ≤5 CI = 1.05–1.14). assessed by the gate the relation be- h/sleep and 5–7h/- Overall conclusion: Newcastle-Ottawa tween sleep duration sleep, 7-8 h/sleep Short sleep duration Scale (NOS) and risk of obesity were mostly consid- is a significant risk Results: The included ered as the reference factor for obesity, studies were category. especially when considered to be of Definition of chronicity sleeping 7 h/night. high quality. Scores not reported. For each 1-h ranged from 6 to 8; decrease in sleep the mean score was duration, the risk of 6.9 (maximum score: 9). obesity increases 9%. All experimental studies and results from longer sleep duration were excluded from the numbers reported in this table Quality assessment only reported in this table if it was conducted with a systematic quality assessment instrument in the review 146 Curr Sleep Medicine Rep (2021) 7:129–154 Results categories [25, 26] or listed the different studies and their definitions of short sleep [27–32]. In most reviews, the defi- Twenty-two articles (13,504,230 participants) were in- nition depended on the original articles; however, Chen et al. cluded with 13 studies (1,811,969 participants) on children  divided sleep duration into three categories (shortest, and/or adolescents and 12 studies (11,692,261 partici- shorter, short). Fatima et al.  on the other hand created pants) on adults (three studies included both children/ three groups with different definitions of insufficient sleep, adolescents and adults). The following domains of day- which were based on the recommendations of the Sleep time functioning were affected by insufficient sleep: Health Foundation. Concerning adolescents, most studies weight/obesity (13 studies), diabetes (four studies), cardio- used 6 to 8 h of sleep per night as insufficient sleep [25, 29, vascular diseases (five studies), cognitive functioning 32, 33]. (four studies), emotional problems (three studies), mortal- Although all included reviews aimed to assess the ef- ity (three studies), and general health/quality of life (three fects of insufficient sleep, operationalized by short sleep studies). One study investigated the relationship between duration, on certain outcome variables, not one review sleep duration and suicidality and a few reviews men- provided a clear definition of chronicity, meaning that tioned additional domains (e.g., physical activity, dietary, no review answered the question of how many nights screen time), which could not be assigned to one of the sleep had to be insufficient or reduced. The reviews usu- above-mentioned domains (see Table 1). Overall, the re- ally included studies that referred to an “average” or sults clearly show that the majority of reviews focused on “general” sleep duration measured by a single question physical health problems, with most articles addressing the such as “how many hours of sleep do you usually get?” relationship between insufficient sleep and obesity/weight, which indirectly includes chronicity as it implies that whereas only a limited number of reviews addressed the individuals receive that amount of sleep over a longer effects of insufficient sleep on behavior and cognitive time period. Additionally, some reviews/meta-analyses functioning. The limited number of reviews on these reported that the measurement of usual sleep duration topics may demonstrate a research gap of original studies was indicated “over the last month” (e.g., ), for in- addressing the effects of insufficient sleep on emotional stance, when the Pittsburg Sleep Quality Index (PSQI; and psychological wellbeing as well as on cognitive per- ) was used, or reported the use of sleep diaries, formance. Figure 2a–d present the number of unique stud- which typically measure one or 2 weeks of subjective ies and participants divided into children/adolescents and sleep (e.g., ). Yet other studies reported the use of adults for each domain separately. self-reported insomnia symptoms over a longer period (e.g., ). The only studies that defined some form of Chronic Sleep Reduction chronicity were the few experimental studies that were included in the reviews, such as 6.5 h in bed per night All included reviews provided data on sleep duration, for 5 nights in one study included in the review by meaning that studies gave a definition of short sleep by Chaput et al. . However, this definition was related providing the exact hours of sleep. Although the definition to experimental studies and is therefore excluded from of short sleep duration usually differed depending on the this scoping review. Table 1 provides the detailed results original studies rather than on theoretical grounds, in most of sleep duration for each review separately. reviews on adults, short sleep duration varied in the ma- jority of included studies between 5 and 6 h of sleep per Effects on Outcome Variables night [14–20]. Few reviews used slightly longer sleep du- rations (e.g., 7 h of sleep) as short sleep duration [17, Weight/Obesity 20–22, 23� ]. The only article that additionally included other (actigraphic and polysomnographic) aspects of sleep We included five systematic reviews/meta-analyses on quality was conducted by Aziz et al., who defined weight/obesity in adults (two systematic reviews, three studies sufficient sleep duration as 7 h sleep per night, but also combining meta-analysis and systematic review) that included used sleep efficiency 85%, sleep fragmentation 25, and 47 studies with 1,039,337 unique participants, and eleven sys- proper sleep staging from polysomnography as indicators tematic reviews/meta-analyses on children and adolescents for sufficient sleep. (six systematic reviews, five studies combining meta- The definition of short sleep for children and adolescents analysis and systematic review) that included 152 unique differed from adults. Some reviews on children used sleep studies with 635,160 participants. Magee and Hale pic- durations 9/10 h of sleep per night as an indication for insuf- tured mixed results for the literature on adults when system- ficient sleep [15, 25], whereas others (additionally) specified atically reviewing longitudinal studies: Eight studies reported sleep duration in more detail depending on different age an association between short sleep duration and weight gain Curr Sleep Medicine Rep (2021) 7:129–154 147 Child/adolescent studies mortality general health cognion emoonal problems CVD diabes weight/obesity 0 20 40 60 80 100 120 140 160 Child/adolescent parcipants mortality general health cognion emoonal problems CVD diabes weight/obesity 0 K 200 K 400 K 600 K 800 K 1000 K 1200 K Adult studies mortality general health cognion emoonal problems CVD diabes weight/obesity 0 10203040506070 Adult parcipants mortality general health cognion emoonal problems CVD diabes weight/obesity 0 K 500 K 1000 K 1500 K 2000 K 2500 K 3000 K 3500 K 4000 K Fig. 2 a–d Numbers of unique child/adolescent and adult studies and development, physical activity, and quality of life/wellbeing ; participants included in the reviews and meta-analyses. NB. some out- suicidality ; insulin resistance, sedentarism, unhealthy dietary pat- come or mediator variables from the following articles could not be in- terns, physical activity, screen time, change in ghrelin, and leptin levels cluded in the categories of the graphs: adiposity, emotion regulation, ; inflammation ; sleep, diet, and physical activity  growth, screen time, risk of injuries, cognitive development, motor 148 Curr Sleep Medicine Rep (2021) 7:129–154 whereas five studies did not. Nielsen and colleagues in- analysis on prospective studies that short sleep duration seems cluded both reviews of studies on adults and children from to be a risk factor or marker for the development of obesity in before 2008 as well as unique new studies published after infants, children, and adolescents; however, studies per age 2008 and which were not part of the previous reviews. They group were rather small with three to eight studies per age concluded that in children and young adults, short sleep dura- group. Furthermore, the authors demonstrated that sleep dura- tion is consistently related to increased risk of obesity, but not tion was related to significant BMI changes. Li et al.  in older adults. This finding supports the above-summarized included only cohort studies in their meta-analysis and report- evidence of adult studies. The meta-analysis of prospective ed that short sleep duration increased the risk for obesity. To cohort studies by Itani et al.  showed a clear effect of summarize, the existing systematic reviews and meta-analyses insufficient sleep on obesity as sleep duration 6 h per night point towards an increased risk of weight gain/obesity in in- was related to a significant increase in obesity. Comparable dividuals with insufficient sleep. effects for cross-sectional studies were reported by Cappuccio et al.  and by Zhou et al. , who showed that the risk of Diabetes obesity increases 9% for each 1-h decrease in sleep duration, when compared with 7 h of sleep per night. We included two articles on diabetes combining a meta- Concerning children and adolescents, the systematic re- analysis and systematic review in adults, which included 29 view on children by Fobian et al.  found the strongest unique studies with 848,011 participants, and two systematic evidence for an association between sleep and obesity, hyper- reviews in children and adolescents, which included 16 tension, and insulin sensitivity, and some evidence of a link unique studies with 27,804 participants. The systematic re- between insulin resistance and sleep. There was no strong view and meta-analysis on cohort studies of adults by evidence of a relationship between glucose or insulin, lipids, Anothaisintawee et al.  reported a significantly small ef- or inflammation and sleep. Similarly, also Nielsen et al. , fect of sleep for diabetes. Comparable results were reported by Felsö et al. , and Chaput et al.  found support for the Itani et al. , who found that a sleep duration 6 h per night idea that insufficient sleep influences weight gain and conse- was associated with a significant increase in diabetes. quently the risk of obesity in children; however, the authors No study directly assessed the association between sleep claim that the underlying explanatory mechanisms (e.g., phys- and diabetes in children and adolescents; however, Felsö et al. ical activity) are still not fully understood. Still, it should be  verified in their systematic review of studies with mixed mentioned that not all included studies reported clear positive study designs that short sleep duration is related to the devel- results. The conclusion for school-aged children and adoles- opment of insulin resistance, sedentarism, and unhealthy die- cents by Chaput et al.  was for instance based on 58 stud- tary patterns. Similarly, Fobian et al.  reported a strong ies with significant associations and 13 studies reporting null relationship between sleep and obesity, hypertension, and in- findings. A similar effect was found in younger children aged sulin sensitivity, and some evidence of a link between insulin 0–4years , which was based on 20 studies reporting a resistance and sleep. There was no strong evidence of a rela- positive association, nine studies with null findings, and two tionship between glucose or insulin, lipids, or inflammation studies with an association between long sleep duration and and sleep. It can therefore be concluded that short sleep seems adiposity. Based on the review of longitudinal studies, Magee to increase the risk to develop diabetes or health conditions and Hale  furthermore concluded that short sleep in chil- that are related to diabetes, in all age groups. dren and adolescents is consistently related to weight gain. There was only one exception in one study that included older Cardiovascular Disease children (8.0–12.9 years at baseline and followed for 5 years). The meta-analytic evidence, which is based on cross-section- We included two articles on cardiovascular disease in adults al, cohort, and case-control studies, by Chen et al. showed (one systematic review, one study including a systematic re- that compared to children receiving sufficient sleep, children view and meta-analysis), which included 56 unique studies who havemuchshorter sleep durationalsohaveanincreased with 1,260,193 participants, and three systematic reviews in risk of obesity. Intriguingly, the group of children with the children and adolescents, which included 32 unique studies shortest sleep duration even had the highest risk of obesity. with 51,855 participants. Aziz et al.  conclude that in Similar results were found by Cappuccio et al. , who re- cohort studies with adults, a relationship of sleep with in- ported an increased risk of obesity for children with short creased subclinical cardiovascular disease burden; however, sleep duration. Fatima et al.  showed in a meta-analysis it has to be considered that mixed results were reported by the of longitudinal studies that children and adolescents with short included studies. Itani et al.  were the only authors who sleep double their risk to develop obesity compared with those conducted a meta-analysis and indicated a threshold of sleep who sleep longer with a stronger association for adolescents duration, by stating that a sleep duration 6 h per night was than for children. Miller et al.  showed in their meta- associated with a significant increase in cardiovascular Curr Sleep Medicine Rep (2021) 7:129–154 149 diseases. Mixed findings for sleep and cardiometabolic performance seems to be small and complex as the results of markers in children and adolescents aged 5–17 years were the only meta-analysis highlight the need to carefully differ- shown by Chaput et al.  as some studies being included entiate between different cognitive domains. in their review found a positive association whereas others reported null findings. In another review from Chaput et al. Emotional Problems on children aged 0–4 years, the authors found no studies that investigated the association between sleep duration and We included one meta-analysis on emotional problems in cardiometabolic biomarkers. Fobian et al.  found in their adults that included two unique studies with 10,096 partici- systematic review on children, in which mixed study designs pants, and two systematic reviews in children and adolescents were included, the strongest evidence of an association with that included unique 81 studies with 554,978 participants. The sleep for obesity, hypertension, and insulin sensitivity, and study by Itani et al.  was the only meta-analysis on studies some evidence of a link between insulin resistance and sleep, with adults that aimed to include depression as the outcome whereas there was no strong evidence of a relationship be- variable. However, the authors were unable to conduct the tween glucose or insulin, lipids, or inflammation and sleep. analysis as the amount of available evidence was too small. In summary, although the existing reviews suggest a rela- Therefore, no conclusion about the association between sleep tionship between short sleep duration and cardiovascular dis- and emotional problems in adults can be drawn. Based on 49 ease, it seems to be rather small and complex. More research studies reporting that long sleep duration was associated with focusing on this complexity should therefore be conducted better emotion regulation, eleven studies with null findings and compared in systematic reviews and/or meta-analyses. and two studies with opposite findings, Chaput et al.  conclude that insufficient sleep is related to more emotion Cognitive Functioning regulation problems in children and adolescents aged 5–17 years. A similar relationship was found in a review on younger We included two articles on cognitive functioning in adults children aged 0 to 4 years, which was based on 13 studies (one systematic review and one study including a systematic reporting a significant effect, ten studies reporting null find- review and meta-analysis), which included 37 unique studies ings, and two studies that found an association between longer with 124,696 participants, and two systematic reviews in chil- sleep duration and poorer emotional regulation . It re- dren and adolescents, which included 42 unique studies on mains an interesting research gap that no results from a sys- cognitive functioning with 57,993 participants. Brewster tematic review or meta-analysis in adults were available and et al.  included cross-sectional, prospective, and longitu- that the empirical evidence in children and adolescents is rath- dinal studies in their systematic review and found mixed re- er limited. sults with indications of a U-shaped relationship between sleep and cognitive functioning in older adults. The meta- General Health/Quality of Life analysis by Lo et al. , which included cross-sectional and prospective cohort studies, found a small effects size for We included one systematic review on general health in adults the relationship between sleep duration and cognitive func- that included five studies with 159,044 participants, and two tioning. The effect differed depending on cognitive domains, systematic reviews on children and adolescents that included as insufficient sleep was significantly associated with poorer four studies with 151,834 participants. Concerning general performance in multiple-domain tasks, executive functioning, health in adults, Frange et al.  found in their systematic verbal memory, and memory capacity but not with speed of review on mainly cross-sectional studies that sleep duration processing. Chaput et al.  included studies on children and negatively affects self-reported health even when analyses adolescents and Chaput et al.  even investigated the rela- were adjusted for possible confounders. However, as men- tionship between sleep duration and cognitive development in tioned above, this systematic review was based on only five children aged 0 to 4 years. Both reviews, including mixed studies and should therefore be carefully interpreted. study designs, found mixed results for the association between Chaput et al.  conclude in their review on school-aged sleep duration and cognition, which is in line with the reviews children and adolescents that an association between sleep on adults. Academic achievement, which is closely related to duration and quality of life exists, whereas the evidence for cognitive functioning, on the other hand, was found to be the relationship between sleep duration and harm/injury was critically related to shortened sleep . Interestingly, none mixed. However, it has to be mentioned that the conclusion on of the reviews included a clear cut-off indicating sufficient quality of life could only be based on three studies and that all sleep, which would allow drawing conclusions on the amount reported an effect and the conclusion on harm/injury was and period of insufficient sleep needed in order to significantly based on four studies, with one study reporting a positive affect cognitive functioning. It can therefore be concluded that effect, one study reporting no effect, and two studies reporting the relationship between insufficient sleep and cognitive mixed results. In younger children, aged 0 to 4 years, the 150 Curr Sleep Medicine Rep (2021) 7:129–154 review by Chaput et al.  found that growth, risk of injuries, effect for subgroups. In response to this, many of the included and screen time are affected by shortened sleep, whereas re- meta-analyses used these variables to control for possible con- sults seem to be less clear for motor development, physical founding effect and to identify the pure effect of sleep duration activity, and quality of life. This may at least partially be and quality on health outcomes. In some cases, this led to caused by the very limited number of studies on these topics, including subgroup analyses based on age (e.g., [15, 16, 18, which makes it difficult to compare study results. 22, 23, 25, 26, 32, 36]) and/or gender (e.g., [16, 18, 21–23, These results show that little research has been done inves- 26]), for which results have been described above. However, tigating the relationship between insufficient sleep and general from the included meta-analyses, it is not possible to draw health, both in adults and children. This includes the number direct conclusions on health disparities between populations of studies that could be included in the reviews as well as the with different socio-demographic characteristics, such as number of existing reviews with not one meta-analysis. race/ethnicity, income, and educational level, as health dispar- ities were not the primary aim of the meta-analyses. It also has Mortality to be mentioned that studies often controlled for influential factors, meaning that the meta-analyses were using effects that We included three meta-analyses on the association between were already controlled for possible confounding variables in sleep and mortality in adults that included 62 studies with the original studies. 3,374,285 unique participants. No systematic review/meta- analysis on children or adolescents was found. Based on the review of prospective cohort studies by Gallicchio and Discussion Kalesan , it can be concluded that short sleepers are at higher risk of mortality. Similar results from prospective co- The results from our scoping review regarding the aim to hort studies were reported by Itani et al. , who even report provide an overview of chronic sleep reduction show that a that a sleep duration 6 h/night was associated with a signifi- clear definition of chronic sleep reduction is lacking. None of cant increase in mortality (as well as in diabetes, cardiovascu- the included reviews/meta-analyses explicitly addressed the lar disease, and obesity). Liu et al.  found in their meta- question of chronicity. Studies usually reported an average analysis on prospective cohort studies a J-shaped association sleep duration or defined insufficient sleep on average (e.g., between sleep duration over 24 h (including daytime sleep) 7 h per night) and measured sleep duration with one single and all-cause mortality. They reported that every hour of re- question (“e.g., how many hours do you usually sleep at duced sleep increased the risk of all-cause mortality. night?”). Regarding the second and third aims of this scoping However, concerning night-sleep duration only, less than 7 h review, namely to summarize the effects of short sleep on did not increase the risk of all-cause mortality. daytime functioning and identify research gaps, the results showed that short sleep duration is related to obesity and dia- Study Quality betes. Relations were also demonstrated with CVD, cognitive functioning, emotional problems, general health, and mortal- Out of the 22 included reviews/meta-analyses, only 12 report- ity, but the effects were mostly small, the relations appeared ed on the quality of the included studies (see Table 1 for a complex (e.g., for CVD and cognitive functioning), and the detailed overview). The results were heterogeneous, ranging numbers of studies were small or completely lacking (e.g., from studies with very low to high quality. However, reviews mortality in children/adolescents, and emotional problems in used different assessment tools, to measure study quality. As adults). these tools also focused on different aspects of study quality Although the lack of clearly defined criteria for chronicity (e.g., study design, validity and reliability of measurements, in all studies seems surprising at first sight, the following follow-up measures), the reported results are difficult to com- research challenges may help to explain this result: most stud- pare. Still, we highly recommend including quality assess- ies that were included in the reviews/meta-analyses had cross- ments in reviews/meta-analyses, as it helps to interpret the sectional designs, in which participants were simply asked to described results and conclusions. indicate their average sleep duration. More complex or objec- tive aspects of sleep were not included, and therefore, the Subgroup Analyses and Health Disparities systematic reviews or meta-analyses were unable to address the question of chronicity (e.g., time period during which in- Literature indicates that socio-demographic variables, such as sufficient sleep has to occur to affect functioning) or even the age, gender, income, educational level, and race/ethnicity, can direction of the relations between sleep and other outcomes. influence the duration and quality of sleep. In order to assess Furthermore, it is striking that in many instances the original health disparities, a meta-analysis rather than a systematic studies that were included in these reviews/meta-analyses did review is needed as this would allow an estimation of the exact not report criteria for chronicity either. Considering the fact Curr Sleep Medicine Rep (2021) 7:129–154 151 that in experimental studies a dose-response relationship be- Concerning the outcomes of daytime functioning related to tween sleep and performance has been demonstrated (e.g., short sleep duration, the results of this scoping review show ), this lack of definitions and reporting of the time period that most reviews addressed weight gain/obesity both in of sleep duration is an important omission in the current liter- children/adolescents and in adults, and also found the most ature. In some reviews/meta-analyses, studies were included robust association, indicating that insufficient sleep is related that measured sleep with sleep logs and/or actigraphy over 7 to weight gain and/or even obesity. Some reviews attempted days, or used polysomnography. These measures of sleep du- to identify possible mediators (e.g., dietary, physical activity, ration may be more reliable than a single self-report item in a screen time) that may play a role within this association. long questionnaire, but it is unclear how these measures relate Although no strong evidence of these factors could be con- to chronicity, meaning how sleep duration from for example firmed by the reviews so far, the existing empirical evidence is polysomnography is related to usual sleep duration over a limited and future studies should further investigate their role longer time period. Another important issue refers to the phe- in order to understand the complexity of the effects of insuf- nomenon of individual sleep need, which can be defined as the ficient sleep. The effects on diabetes and CVD seem to be less amount of sleep somebody needs without being influenced by stable, as especially mixed results were found for the relation- previous sleep debt . Individual sleep need was shown to ship between sleep and the latter. Mortality and general health range from 7.5 to 8.5 h/sleep per night  with an average seem to be related to sleep reduction; however, again the sleep need of 8.16  for adults, and 9.35 h for adolescents available empirical evidence is rather limited and reviews [40� ], and has a genetic component with 31% to 55% herita- were often based on the results of only a few studies. bility [41–43]. From this existing evidence, it can be conclud- Moreover, the largest body of literature consists of large ed that the effects of sleep duration should be interpreted with cross-sectional studies that is limited for causal inference, reference to ones’ individual sleep need; however, this facet of and mostly used single-item self or parent report of sleep sleep was not addressed in the included reviews/meta- duration. The limitations of these methods have been outlined analyses. above, and results should be interpreted with caution regard- One attempt to overcome the problem of the influence of ing direction of the risk, and validity of the measurements. individual sleep need on optimal sleep duration has been pro- Although the literature often postulates that sleep affects vided by Meijer , who developed the “Chronic Sleep cognitive functioning [47–49], our review points towards Reduction Questionnaire (CSRQ)” for adolescents [44, 45]. mixed results and rather small effect sizes. This may especial- This questionnaire measures daytime problems rather than ly be caused by the finding that sleep seems to affect certain sleep directly and assumes that adolescents only score high aspects of cognitive functioning (e.g., multiple-domain tasks, on these problems if they experience less sleep than needed executive functioning, verbal memory, memory capacity), over a prolonged time period. Although this seems to be an which refer to higher order cognitive functioning, whereas interesting approach, also this questionnaire does not give an more basic cognitive aspects (e.g., speed of processing) seem evidence-based rationale for the time period of 2 weeks, dur- to be unaffected. This finding is in line with other reviews that ing which these daytime symptoms have to occur. In contrast, found sleep to be related to some, but not all areas of cognitive the DSM-5 defines that symptoms of insomnia must be pres- functioning [50, 51] (these reviews were not included in this ent for at least 3 months to fulfill the criteria of the diag- scoping review as they did not meet the inclusion criteria). nosis “chronic insomnia” similar to the ICSD-3 classi- Reynoud et al.  furthermore found more significant effects fication of “Chronic Insomnia Disorder” . However, for studies with larger sample sizes, which supports the idea the DSM-5 also claims that individuals experience impair- that effect sizes seem to be rather small and large sample sizes ments in their daytime functioning , indirectly indicat- are needed to detect effects and to ensure sufficient statistical ing that individuals sleep less than their individual sleep power. Interestingly, also academic achievement, which re- need. Although this idea is in line with the assumption of quires cognitive functioning, seems to be related to shortened the CSRQ, based on the results of this scoping review, it sleep. This finding was also supported by two meta-analyses remains unclear whether the CSRQ subscales (shortness in children and adolescents [52, 53]; however, it has to be of sleep, sleepiness, irritation, loss of energy) represent mentioned that also these meta-analyses reported small effect the best empirically supported categories. We found that sizes, indicating that additional factors seem to play an impor- especially physical health was related to insufficient sleep, tant role. We highly recommend focusing future research on whereas the evidence for the relationship between sleep such mediators within the long-term relationship between and emotional or psychological wellbeing was too limited sleep and cognitive functioning and/or academic achievement. to draw clear conclusions (see discussion below). Surprisingly little evidence was found for the relationship be- Furthermore, we did not find reviews on the effects of tween sleep and emotional problems, although experimental insufficient sleep on daytime sleepiness, one of the studies and clinical groups point towards an interplay between CSRQ subscales. sleep problems, mood, and psychiatric problems . It was 152 Curr Sleep Medicine Rep (2021) 7:129–154 even shown that sleep problems are already present prior to sleep and it was therefore impossible to provide a def- the development or recurrence of depression  and Beatti inition of chronic sleep reduction, which (as stated in et al.  conclude in their systematic review that sleep dep- the introduction) is highly needed for future research rivation affects emotion recognition, measured with experi- and clinical practice. mental paradigms. These effects were not only seen on a be- havioral or emotional level but also affected brain activity , pointing towards underlying neural correlates. The pres- Future Directions and Research Agenda ent scoping review therefore identifies an important research gap in the area of sleep and emotional problems in healthy We highly recommend more studies that aim to define the samples, which should be addressed in future studies, optimal- chronicity of insufficient sleep. This first attempt should be ly in (prospective) cohort or longitudinal study designs. to experimentally clarify the dose-response relationship be- Overall, the study quality of included studies was rather tween insufficient sleep and outcome variables, followed by low for all outcome variables. Therefore, more high-quality more naturalistic, quasi-experimental studies in individuals’ research should be conducted in the future in order to draw daily life and natural environment. Additionally, influential valid conclusions about the effects of insufficient sleep on factors, including mediating, and moderating effects, and daytime functioning. The finding that the majority of health disparities between different socio-demographic popu- reviews/meta-analysis investigated weight gain/obesity, lations, should be assessed. We also recommend focusing whereas cognitive functioning, for example, appeared in much more on resilience and protective factors that may buffer the fewer numbers, especially if compared in numbers of partici- negative effects of insufficient sleep. More knowledge on pants that were included in the studies, could be partially due these topics is highly relevant for future prevention and inter- to practical and financial constraints that are inherent to mea- vention studies that aim to improve daytime functioning and suring cognitive functioning in large cohorts. However, the physical and/or psychological health by improving individ- inherent complexity of measuring cognitive functioning could uals’ sleep. Regarding outcomes, we recommend further re- be argued to be present as well in measuring sleep. As search on areas that have been demonstrated to be affected by discussed above, sleep duration in most reviews/meta- sleep reduction in experimental research, but are largely lack- analyses was measured with single-item questions, which dis- ing in cohort studies, such as emotional and behavioral prob- regards chronicity, quality, variability, fragmentation, timing, lems, and cognitive functioning. Furthermore, research should and individual sleep need. Moreover, measuring weight gain/- focus to develop more specific and clear evidence-based obesity requires more than a single item on a questionnaire guidelines on chronic sleep reduction and its effects, and de- and can be regarded as a somewhat labor-intensive measure. It velop validated measures that can replace the existing seems that especially in larger cohort studies that adopt such methods in larger epidemiological or cohort studies. single-item questions on sleep duration, this complexity has Such guidelines would be helpful for future research, been underestimated. It is therefore highly recommended to as they would help to make studies more comparable adjust the method of sleep measurement in epidemiological and guide clinical practitioners. studies, which is in concurrence with previous recommenda- tions [57, 58]. Supplementary Information The online version contains supplementary material available at https://doi.org/10.1007/s40675-021-00214-1. Limitations Acknowledgements The authors wish to thank Janneke Staaks for her The following limitations should be mentioned: (1) there is a assistance in the systematic search. big heterogeneity not only between original studies but also Code Availability Not applicable between the included systematic reviews and meta-analyses, which makes it difficult to directly compare the results; (2) Author Contribution Both authors contributed equally to the manuscript many studies measure sleep duration with one single item, and study. which may lead to biased results; (3) the study quality of most studies was rather low; (4) as we included only previous sys- Availability of Data and Material Not applicable tematic reviews or meta-analyses rather than original studies, lacking evidence on some outcome domains Compliance with Ethical Standards (e.g., behavioral problems, emotional problems) may ei- ther result from a small number of original studies, Conflict of Interest The authors declare that they have no conflict of interest. representing a research gap in this area, or from a lack of reviews on this topic; (5) the results clearly demon- Human and Animal Rights and Informed Consent Not applicable. strate a lack of definitions for chronicity of insufficient Curr Sleep Medicine Rep (2021) 7:129–154 153 Consent for Publication The authors agree to the terms and conditions 12.� Matricciani L, Paquet C, Galland B, Short M, Olds T. 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Current Sleep Medicine Reports – Springer Journals
Published: Sep 1, 2021
Keywords: Sleep reduction; Sleep deprivation; Sleep problems; Daytime consequences
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