Associations of screen time, sedentary time and physical activity with sleep in under 5s: A systematic review and meta-analysis

1Total time spent asleep over 24-hours (including naps if this was included in the original study). A longer total sleep duration was treated as favorable.

2Frequency of child waking up during the night. Less night awakenings were treated as favorable.

3Length of time for a child to transition from full wakefulness to sleep after “lights out”. Shorter transition time from full wakefulness to sleep after lights out was treated as favorable.

4Time a child is put to bed. An earlier bed time was treated as favorable.

5Child naps during the day (yes/no). Child napping was treated as favorable.

6Percentage of total sleep duration spent asleep after sleep onset. A higher percentage of time spent asleep after sleep onset was treated as favorable.

7Score based on stable average sleep duration. More stable sleep duration was treated as favorable.

8Combination score based on different sleep outcomes (e.g., bed time, number of night awakenings, sleep onset latency). Better sleep quality was treated as favorable; this classification was driven by the reporting of included papers.

9Adequate sleep plays a critical role in children's health and development, particularly in the early years. Short sleep duration in preschool children is linked to obesity in later childhood. Furthermore, sleep problems beyond age two are associated with reduced grey matter volume at seven years, indicating a role of sleep in early brain development.

10International guidelines recommend that infants (0–1 y) sleep for up to 17 h/d, while toddlers (1–3 y) and preschoolers (3–5 y) should sleep between 10 and 14 h/d. However, today's children sleep less than they did a century ago and 20–30% of parents report that their child has difficulties falling or staying asleep. The causes for this apparent epidemic of sleep problems are likely multi-factorial but lifestyle changes in an increasingly digitized world are a cause for concern.

11Australia, Canada, South Africa, New Zealand and WHO have issued 24-h movement guidelines for under 5s, recommending an ‘optimal day’ in terms of children's sleep, physical activity and sedentary behaviors (including screen time). This ‘whole day matters’ approach places each behavior along a continuum, where declines in one behavior results in an increase in another. Studies in older children and adults have shown that daytime physical activity and screen time both influence sleep, but less is known about these relationships in children under 5 y of age. The early years are also a critical period in life for establishing healthy behaviors as screen time and physical activity appear to track from early into later childhood and adolescence and consequently may influence sleep later in life.

12No reviews to date have synthesized and evaluated the quality of international research evidence in the under 5s. This review therefore sought to determine how screen time, sedentary time and physical activity are associated with eight sleep outcomes (i.e., total sleep duration; night awakenings; sleep onset latency; bed time; daytime napping; sleep efficiency; sleep stability; and sleep quality) in children aged 0–4 y.

13This systematic review was conducted and reported according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). A systematic literature search was undertaken in April 2018 and updated in March 2019, using search terms related to: population; study design; outcome; exposure; and exclusion of clinical populations (Appendix 1). The search was conducted in 17 electronic databases: EBSCO (CINAHL); Cochrane Library (CENTRAL); OVID (EMBASE, MEDLINE, PsycINFO) and Web of Science (all databases). Citations were downloaded into Endnote citation management software (Thomson Reuters, Philadelphia, PA, USA) and de-duplicated. Included papers were searched for additional relevant publications, as were relevant reviews. No language or publication date restrictions were placed on the search.

14Studies were included if they: 1) reported results from a cross-sectional, longitudinal or experimental study and 2) assessed the relationship between screen time (total daily screen time; evening screen time) or any movement behavior (i.e., sedentary time; total, light, moderate-to-vigorous physical activity; floor-based play (infants); outdoor play/time; sports participation) and any sleep outcomes reported. Studies assessed healthy children (i.e., general populations, including those with overweight/obesity) aged birth to 59 mo at baseline; objective or subjective measures of exposures and outcomes were considered. Exclusion criteria included: 1) clinical populations (e.g., children with chronic health conditions e.g., asthma, or developmental disorders e.g., cerebral palsy, autism) 2) qualitative studies; 3) studies assessing screen-based content; and 4) those assessing electromagnetic radiation.

15Identified titles and abstracts were screened for relevance (KH, GK) and included titles were separated by exposure type (sedentary time, physical activity or screen time; KH). Full texts of identified articles were retrieved and read in full to assess eligibility for inclusion (physical activity: KH, RK; sedentary behaviors: XJ, AM). Reviewers independently extracted and cross-checked relevant data using a pre-piloted data extraction form (physical activity: AM, KH; sedentary behaviors: AH, CH, XJ). Data were extracted per age group; infants (0 to <1 y), toddlers (1 to <3 y), preschoolers (3 to <5 y) and for each exposure-outcome association. The split between age groups was chosen for two reasons. First, major developmental differences exist during the early years in both physical and cognitive development. Therefore, we hypothesize that the investigated associations may be different for each of these age groups. Second, the chosen split in age groups is consistent with the international 24-h movement guidelines.

16Investigated exposures were: 1) daily and evening screen time including parent report of child time spent on TV, tablet, phone, playing computer games, using the internet; 2) accelerometry measured physical activity including total sedentary behavior, light physical activity and moderate-to-vigorous physical activity; 3) parent reported floor based play, organized sport and outdoor play. Total sedentary behavior and screen time were treated as two different exposures to provide more detailed evidence about whether screen time or all sedentary behaviors influence sleep.

17For longitudinal studies, all time points up to age five were included. The latest time point included was before, or as soon after, the children were five years old (>5 y if no follow-up data on <5 y was provided). If two or more papers reported on the same study sample, both were treated as separate studies if they reported different exposure-outcome relationships (n = 4). Several papers examined multiple exposure-outcome associations (e.g., how total screen time and TV time were associated with sleep) and reported findings for different groups (e.g., examined differences across age groups, by time of the week, or by sex). Each exposure-outcome was therefore counted as an individual association, e.g., a paper examining the association between screen time and total sleep duration, but reporting results for weekdays and weekend days separately, was counted as one study but two associations. For experimental studies, differences in outcomes between control and intervention groups over time were used to assess influence of exposures. Where possible, results from adjusted multivariable models were reported.

18Reviewers who extracted the data also assessed the methodological quality of primary studies and any discrepancies were resolved by consensus. Risk of bias assessment was completed as part of the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) of evidence quality. Six domains (selection, performance, detection, attrition, reporting, and other sources of bias) specific to study design (observational or experimental) were assessed. Each domain was determined to have a low, unclear or high risk of bias.

19Due to the heterogeneous nature of included studies, and the range of exposures and outcomes assessed, meta-analysis was only appropriate for one exposure-outcome association total screen time and sleep duration in infants, toddlers and preschoolers. Where available, correlation coefficients were recorded for each study. If studies did not report correlations coefficients but provided beta coefficients these were converted in to correlation coefficients using the method described by Peterson and Brown (2005). Only studies reporting cross-sectional associations were included in the main analysis. A sensitivity analysis including longitudinal outcomes was conducted but no significant differences were found between the two models. Data were pooled in a random-effect meta-analyses using Comprehensive Meta-analysis, version 3.3.07. Heterogeneity across studies was assessed using I2 statistics (I2 of 0–40% represents low heterogeneity and 75–100% considerable heterogeneity).

20For the remaining associations, as recommended by the Cochrane handbook for systematic reviews of interventions, vote counting based on the direction of association was conducted, comparing the number of favorable to unfavorable associations. Favorable associations were categorized as those where the exposure measure resulted in a positive association with sleep outcomes (e.g., less screen time associated with longer total sleep duration). Associations were unfavorable if the exposure measure resulted in a negative association with sleep (e.g., more screen time associated with shorter total sleep duration). Summary results per exposure-outcome association were presented as number of unfavorable (for screen time and sedentary behavior) and favorable (for physical activity, outdoor play and sport club attendance) associations divided by the total number of studies included. A binomial probability test was conducted. The p-value from this test indicates the probability of observing the summary results if the exposure-outcome associations were in the opposite direction, thus a small p-value indicates a higher probability the results are valid. This method does not rely on p-values reported by the authors of the primary studies.

21GRADE was performed on all findings, stratified according to child age (infants; toddlers; preschoolers), exposure, and outcome measure, with possible range from very low to high. GRADE assigns an initial rating to each study design (i.e., high for randomized controlled trials, low for observational studies - both longitudinal and cross-sectional). This was then upgraded or downgraded according to the risk of bias, inconsistency, indirectness, imprecision, publication bias, dose–response relationship, residual confounding or the size of the magnitude.

22Initial searches identified 1604 articles, 90 full-text articles were screened, and of these, 31 studies (29 unique cohorts) met the inclusion criteria and were included in the data analysis (Fig. S1). A total of 60,445 children were included, ranging from 22 to 39,813 participants per article (age range 0–4.99 y). Included articles were published between 2007 and 2019 and conducted in North America (n = 10); Europe (n = 7); Asia (n = 8); Australasia (n = 5); and one article included participants from multiple countries.

23One article reported an experimental design (RCT;), seven were longitudinal (of which four also analyzed data cross-sectionally) and 23 were cross-sectional. Eleven articles examined the association between physical activity and sleep, five articles examine the association between sedentary time and sleep and 23 articles examined the association between screen time and sleep. Eight articles had an age range covering more than one age group (infants and toddlers n = 4; toddlers and preschoolers n = 2; infants, toddlers and preschoolers n = 1; Table 1).

24The relationship between total daily screen time and sleep was examined in 20 studies (infants n = 4; toddlers n = 9; preschoolers n = 16; Table 2). In infants, higher levels of total daily screen time were associated with shorter total sleep duration (3/5 associations), more night awakenings (1/1) and longer sleep onset latency (1/1). Toddlers' and preschoolers' total daily screen time was unfavorably associated with sleep outcomes in 39/43 associations. In toddlers, higher levels of total daily screen time were associated with shorter total sleep duration (7/8), more night awakenings and later bedtime (2/2), longer sleep onset latency (3/3), lower sleep quality (1/1) and worse sleep stability (1/1). In preschoolers, higher levels of total daily screen time were associated with shorter total sleep duration (12/13), more night awakenings (2/3), later bedtime and lower sleep quality (3/3), longer sleep onset latency (2/2), and less daytime napping (1/2). Few studies reported favorable associations between higher levels of total screen time and any sleep outcomes (n = 6 associations).

25A subset of seven studies were included in the random-effect meta-analysis to quantify the effect of total screen time on sleep duration (Fig. 1). The pooled correlation coefficient was −0.09 (95% CI: −0.17, −0.01; I2 = 90.0%; p = 0.04). Sub-group analysis showed similar results for infants (r = −0.07; 95% CI: −0.12, −0.03; I2 = 0.00%; p = 0.002) and toddlers (r = −0.13; 95% CI: −0.21, −0.04; I2 = NA; p = 0.004). However, in preschoolers the effect of total screen time on sleep duration became non-significant (r = −0.10; 95% CI: −0.25, 0.05; I2 = 93.5%; p = 0.203).

26The relationship between evening screen time and sleep was examined in eight studies (infants n = 3; toddlers n = 4; preschool-aged children n = 4; Table 3). In infants, higher levels of evening screen time were associated with shorter nighttime sleep duration (2/2) and later bedtime (1/1). Toddlers' and preschoolers' evening screen time was unfavorably associated with sleep in 8/9 associations. In toddlers, higher levels of evening screen time were associated with shorter total sleep duration (2/2) and later bedtime (1/1). In preschoolers, higher levels of evening screen time were associated with shorter total sleep duration (3/4), later bedtime and lower sleep quality (1/1). Importantly, only one study reported a favorable association between evening screen time and any sleep outcome.

27The association between total sedentary time and sleep was examined in five studies (infants n = 2; toddlers n = 0; preschoolers n = 3; Table 4). In infants, higher levels of total sedentary time were associated with shorter sleep time duration (2/2), more night awakenings (1/1), less daytime napping (1/2) and better sleep efficiency (1/1). In preschoolers, higher levels of total sedentary time were associated with shorter sleep time duration (1/2) associations and later bedtime (1/1). More sedentary time was associated with fewer night awakenings (1/1). A decrease in sedentary time showed an association with improved sleep quality (1/1). No evidence was available for toddlers.

28The association between physical activity related behaviors (i.e., total physical activity, moderate-to-vigorous physical activity, outdoor play and sports participation) and sleep was examined in 11 studies (infants n = 1; toddlers n = 3; preschoolers n = 7; Table 4). The relationship between total physical activity and sleep was examined in five studies. In infants, higher levels of total physical activity were associated with shorter total sleep duration, worse sleep efficiency and less daytime napping (1/1). In toddlers, higher levels of total physical activity were associated with longer total sleep duration (1/2), better sleep quality (2/2) and better sleep stability (1/1). In preschoolers, higher levels of total physical activity were associated with shorter total sleep duration and more night awakenings (1/1) and better sleep stability (1/1).

29Seven studies assessed the relationship between physical activity intensity and sleep. In one study, conducted in preschoolers, light physical activity was associated with later bedtime (1/1 association). In toddlers, higher levels of moderate-to-vigorous physical activity were associated with better sleep quality (1/2), and better sleep stability (1/1), and shorter total sleep duration (1/1). In preschoolers, higher levels of moderate-to-vigorous physical activity were associated with shorter total sleep duration (1/2) and later bedtime (1/1), better sleep quality and better sleep stability (1/1). No evidence was available for infants.

30The relationship between floor-based play was examined in one study for infants: floor-based play was associated with shorter total sleep duration, less daytime napping (1/1) and better sleep efficiency (1/1). The relationship between time spent playing outdoors and sleep was examined in two studies. Toddlers' outdoor play was associated with shorter total sleep duration (2/2), shorter sleep onset latency, fewer night awakenings and earlier bedtime (2/2). Preschoolers' outdoor play was associated with longer total sleep duration (1/2) and fewer night awakenings, earlier bedtime and shorter sleep onset latency (1/1). Preschoolers' attendance at sports clubs was associated with earlier bedtime and better sleep efficiency (i.e., higher fraction of total sleep spent asleep after sleep onset; 1/1).

31The quality of evidence ranged from very low to moderate for moderate-to-vigorous physical activity; and very low for all other exposure-outcome associations across age groups (Table 2, Table 3, Table 4). Most studies were downgraded due to a serious risk of bias (commonly due to use of exposure or outcome measures with unknown psychometric properties; Table S1).

32To our knowledge, this is the first systematic review to explore how screen time and movement behaviors are associated with sleep in children under 5 y. This review highlighted a trend for an unfavorable association between higher levels of total daily and evening screen time and sleep outcomes in infants, toddlers and preschoolers; very few studies showed favorable screen-sleep associations. Meta-analysis conducted in a sub-sample of studies to examine the association between daily screen time and sleep duration confirmed these unfavorable associations in infants and toddlers. In preschoolers, the meta-analysis did not show a significant association, but this may be due to the heterogeneity of the included studies. Evidence for associations between total daily sedentary time/physical activity and sleep was less conclusive: there was an indication that more outdoor play and higher levels of moderate-to-vigorous physical activity were favorably associated with sleep outcomes in toddlers and preschoolers. Most evidence was from observational studies (both cross-sectional and longitudinal), did not show significant associations, and did not report on dose–response relationships leading to evidence frequently being classified as low quality. In addition, no clear differences were found between studies including large (>500) or small (<500) sample sizes.

33Established early in childhood, sleep patterns are governed by a complex interplay of physiological, genetic, psychological and social/environmental factors. A range of behaviors, including physical activity, sedentary and screen time, may delay or displace sleep - ‘5 more minutes please!’ Moreover, socio-environmental influences such as parenting style, the home environment, and socioeconomic status are likely to influence young children's sleep, screen and activity behaviors.

34In line with a recent systematic review in older children (5–20 y), our review and meta-analysis highlights that screen time appears to be unfavorably associated with young children's sleep. Short wavelength (blue/green) light emitted from screens suppresses pineal melatonin secretion, influencing both circadian entrainment (via supra-chiasmatic nucleus signaling) and sleep onset (via the hypothalamic ventrolateral pre-optic nucleus). Although evidence is limited in very young children, differential diurnal rate of melatonin secretion appears to emerge early in development at around 27–41 d of age. Theoretically therefore, evening screen exposure in very young children may not only delay sleep onset on exposure but also potentially cause longer term disturbance to sleep stability. A dim light environment prior to bedtime is likely to be conducive to melatonin secretion, simultaneously promoting earlier sleep onset whilst helping to establish and maintain an optimal circadian rhythm, with less night waking. In addition to the light emitted from screens, the content, its interactivity, and subsequent level of arousal, may also adversely affect sleep.

35Despite a wealth of evidence for a positive association between physical activity and sleep in older age groups, very few studies have examined the association between physical activity and total sedentary time on sleep in children 0–4 y. Our review identified evidence suggesting that more outdoor play and time spent engaged in moderate-to-vigorous physical activity may be associated with better sleep outcomes in toddlers and preschoolers. Although experimental research is largely lacking in children and young people, it has been noted in infants that natural light exposure, particularly during the afternoon, may improve nighttime sleep. Such exposure, as part of children's outdoor play, may help to regulate melatonin secretion and circadian rhythm, encouraging regular sleep onset. Several other physiological mechanisms have also been proposed to explain how higher intensity physical activity may positively influence sleep (albeit in the context of adult sleep). These include: 1) activity triggering an increase in body temperature and subsequent cooling with rest to promote sleep onset, and 2) activity reducing negative arousal states which may otherwise lead to sleep problems. Future experimental studies should determine why and how screen time and movement-behaviors impair and promote sleep respectively. This is particularly important given a number of 24-h movement behavior guidelines have recently been published worldwide, which outline an ‘optimal day’ for children's sleep, physical activity and sedentary behaviors (including screen time). Where the ‘whole day matters’ and each behavior is placed along a continuum, declines in one behavior may feasibly result in an increase in another.

36This review highlights important gaps in the evidence base around screen-based and movement behaviors, and sleep outcomes in young children. The quality of evidence summarized in this review was low and in some instances inconclusive. The variation in results may be due to the wide range of exposure and outcome measures used across studies. Moreover, study quality tended to be downgraded due to use of measurement tools with untested psychometric properties, with 20 out of the 28 articles reporting exposures measured using an unpublished questionnaire/failing to report the questionnaire's psychometric properties (Table S1). Studies included in this review frequently focused on television-based screen time, and did not examine the use of more contemporary screens (e.g., tablets, phones) and/or the type of activities children engaged in while using screens (e.g., watching a movie or talking to grandparents on tablet/phone). With the advances in technology over the last decade, it is important that studies now consider the influence of alternative electronic media and screen-based activities (such as e-readers and tablets) on children's sleep. In addition, it is important studies examine the influence different media content may have on children's sleep (e.g., education v. recreational content) While screen-based technology can positively support learning, neglecting its influence on sleep may paradoxically constrain neurodevelopment in the under 5s.

37While objective measures of sleep duration (e.g., accelerometry) and valid and reliable sleep questionnaires are available, very few studies used either to assess sleep outcomes here (n = 10). Accelerometry is known to poorly differentiate between prolonged sedentary behavior and sleep; included studies using accelerometry all used different methods to estimate sleep and wake periods which could have led to the discrepancy in results. Standardized measurement and analysis procedures of exposure and outcomes would allow consistency and validity across studies. There was also a lack of experimental or intervention studies aiming to improve sleep practices in the early years. Last, as the majority of studies included in this review were cross-sectional, cause and effect could not be established. Thus, it is important to consider a possible reverse pathway, i.e., poor sleepers are more fatigued, resulting in more daily sedentary time and less physical activity. This review therefore highlights evidence gaps including the need to develop and evaluate interventions to improve sleep in young children, especially by reducing screen use before bedtime.

38We applied rigorous review methods, including duplicate assessment at every stage. Given that this review was restricted to published studies, publication bias cannot however be ruled out. All included studies were conducted in high and middle-income countries. Almost half included small sample sizes (15 out of 31 studies had fewer than 500 participants), which may have limited their statistical power to detect significant associations. By using vote counting based on the direction of the effect, we limited the impact underpowered studies may have on the summarized results. Nine exposures and nine outcome measures were used here, thus limiting the use of meta-analysis: where common exposure-outcome associations existed (i.e., for screen time and total sleep duration) meta-analysis was conducted. We defined the absence of daytime napping as an unfavorable outcome here as the majority of studies examining this outcome were in infants (n = 3 out of 4). However, daytime napping has been linked to adverse sleep outcomes such as irregular sleep habits in preschoolers. It is therefore difficult to interpret whether napping is an un/favorable behavior when assessed as an isolated outcome. Due to the co-dependence of movement and sleep behaviors, an increase in one behavior would be expected to result in a decline in another, e.g., if physical activity leads to an increase in sleep duration, sedentary behavior is more than likely to decrease. Future studies would benefit from assessing co-dependent behaviors across a 24-h period. Last, most studies included in this review controlled for common confounders (e.g., age, socio-economic status, sex) but few controlled for characteristics in the home and wider environment which may impact sleep (e.g., chaotic home life, shared bedrooms, noise). Future research should consider a wider range of relevant confounders in order to fully elucidate the relationship between screen time, movement behaviors and young children's sleep.

39Screen time is unfavorably associated with multiple sleep outcomes in infants, toddlers and preschoolers. Conversely, in toddlers and preschoolers more time spent in outdoor play, and in higher intensity physical activity, was associated with better sleep outcomes. There is a pressing need for future research to establish how contemporary screen time (e.g., tablets and e-readers) influences the 24-h equipoise of activity and sleep in young children. Public health initiatives and policies are needed to help parents and educators encourage balanced use of screen-based technologies and positive movement behaviors to promote healthy lifestyles and development in the under 5s.While global and national 24-h movement guidelines suggest that a relationship between screen exposure and sleep exists, the recommendations do not explicitly address the relationship between sleep outcomes and other movement behaviors found here. Future 24-h movement behavior guidelines should therefore consider the findings from this review, which highlight the importance of limiting screen time, especially before bedtime, and providing sufficient exposure to natural daylight, in young children.In a time when electronic media use among young children is becoming the norm, it is crucial to raise public (and particularly parental) awareness about the potential harmful effects exposure to screens may have on a young child's sleep and development.Public health initiatives and policies that stress the importance limiting screen time before bed and the potential benefits of active outdoor play for sleep are warranted. For example, the American Academy of Pediatrics in the USA and the Royal College of Paediatrics and Child Health in the UK recently published guidelines recommending an hour's screen curfew before bedtime in children (https://pediatrics.aappublications.org/content/138/5/e20162591; https://www.rcpch.ac.uk/resources/health-impacts-screen-time-guide-clinicians-parents).This review highlights important gaps in the evidence base around screen-based and movement behaviors, and sleep outcomes in young children.Evidence of the validity and reliability of a broader range of screen time measures is needed, with papers here tending to focus on television-based screen time. In addition, standardized measurement and analysis procedures of exposure and outcomes would allow consistency and validity across studies.It is important that studies start to consider the influence of alternative electronic media and screen-based activities (such as e-readers and tablets) on children's sleep.Very few studies have examined the association between physical activity and total sedentary time on sleep in children 0–4 y. No evidence was available for the association between movement and screen behaviors and several of the sleep outcomes in certain age groups.No studies were identified from lower-middle and low-income countries. These countries are likely to have substantial difference in the home and wider environment, which could influence both exposure and outcomes.

40While global and national 24-h movement guidelines suggest that a relationship between screen exposure and sleep exists, the recommendations do not explicitly address the relationship between sleep outcomes and other movement behaviors found here. Future 24-h movement behavior guidelines should therefore consider the findings from this review, which highlight the importance of limiting screen time, especially before bedtime, and providing sufficient exposure to natural daylight, in young children.In a time when electronic media use among young children is becoming the norm, it is crucial to raise public (and particularly parental) awareness about the potential harmful effects exposure to screens may have on a young child's sleep and development.Public health initiatives and policies that stress the importance limiting screen time before bed and the potential benefits of active outdoor play for sleep are warranted. For example, the American Academy of Pediatrics in the USA and the Royal College of Paediatrics and Child Health in the UK recently published guidelines recommending an hour's screen curfew before bedtime in children (https://pediatrics.aappublications.org/content/138/5/e20162591; https://www.rcpch.ac.uk/resources/health-impacts-screen-time-guide-clinicians-parents).

41While global and national 24-h movement guidelines suggest that a relationship between screen exposure and sleep exists, the recommendations do not explicitly address the relationship between sleep outcomes and other movement behaviors found here. Future 24-h movement behavior guidelines should therefore consider the findings from this review, which highlight the importance of limiting screen time, especially before bedtime, and providing sufficient exposure to natural daylight, in young children.

42In a time when electronic media use among young children is becoming the norm, it is crucial to raise public (and particularly parental) awareness about the potential harmful effects exposure to screens may have on a young child's sleep and development.

43Public health initiatives and policies that stress the importance limiting screen time before bed and the potential benefits of active outdoor play for sleep are warranted. For example, the American Academy of Pediatrics in the USA and the Royal College of Paediatrics and Child Health in the UK recently published guidelines recommending an hour's screen curfew before bedtime in children (https://pediatrics.aappublications.org/content/138/5/e20162591; https://www.rcpch.ac.uk/resources/health-impacts-screen-time-guide-clinicians-parents).

44This review highlights important gaps in the evidence base around screen-based and movement behaviors, and sleep outcomes in young children.Evidence of the validity and reliability of a broader range of screen time measures is needed, with papers here tending to focus on television-based screen time. In addition, standardized measurement and analysis procedures of exposure and outcomes would allow consistency and validity across studies.It is important that studies start to consider the influence of alternative electronic media and screen-based activities (such as e-readers and tablets) on children's sleep.Very few studies have examined the association between physical activity and total sedentary time on sleep in children 0–4 y. No evidence was available for the association between movement and screen behaviors and several of the sleep outcomes in certain age groups.No studies were identified from lower-middle and low-income countries. These countries are likely to have substantial difference in the home and wider environment, which could influence both exposure and outcomes.

45Evidence of the validity and reliability of a broader range of screen time measures is needed, with papers here tending to focus on television-based screen time. In addition, standardized measurement and analysis procedures of exposure and outcomes would allow consistency and validity across studies.

46It is important that studies start to consider the influence of alternative electronic media and screen-based activities (such as e-readers and tablets) on children's sleep.

47Very few studies have examined the association between physical activity and total sedentary time on sleep in children 0–4 y. No evidence was available for the association between movement and screen behaviors and several of the sleep outcomes in certain age groups.

48No studies were identified from lower-middle and low-income countries. These countries are likely to have substantial difference in the home and wider environment, which could influence both exposure and outcomes.

49The authors do not have any conflicts of interest to disclose.