Clinical reviewDelayed school start times and adolescent sleep: A systematic review of the experimental evidence
Introduction
Research conducted over the past four decades has demonstrated that acquiring adequate sleep and maintaining a sleep schedule that is consistent with physiological circadian rhythmicity is a component of normal growth and development during childhood and adolescence [1]. Adequate sleep is needed to achieve optimal mental and physical alertness, daytime functioning, and learning capacity in youth [2], *[3], qualities that are of particular importance in the school setting.
Although guidelines differ in terms of recommendations for total sleep time [4], the National Sleep Foundation released recommendations in February 2015 that school-aged children (6–13 y) and adolescents (14–17 y) obtain at least 9–11 h and 8–10 h of sleep per night, respectively [5]. Yet, by some estimates, only one in 10 adolescents meets these recommendations on weeknights [6], [7]. The high prevalence of sleep restriction may be due to early school start times that conflict with the normal developmental shift in circadian biology that favors phase delay (late morning-late day activities and later bedtimes) during puberty development, as indicated by daily endocrine rhythms [8], [9]. In addition to the physiological underpinnings, evening chronotype preference may be driven by social pressures rooted in involuntary (e.g., staying awake to complete homework) or voluntary (e.g., engaging with social media) actions [10].
The large proportion of adolescents who do not obtain optimal sleep duration [6], [7] is particularly alarming because chronic sleep restriction, defined as partial sleep deprivation, sleep loss, insufficient or deficient sleep, leads to a myriad of health, safety, behavioral, and cognitive and academic deficits. Disrupted sleep-wake cycles and sleep restriction also contribute to pathophysiological effects on the renal, cardiovascular, thermoregulatory, digestive, and endocrine systems [2], [11]. Sleep restriction can lead to insulin resistance and changes in the satiety hormones leptin and ghrelin [12], [13], all mechanisms for the development of metabolic abnormalities, obesity, and type 2 diabetes [14], [15], [16], [17]. Inadequate sleep is also associated with many risky health behaviors, such as lack of physical activity, suicidal ideation, and substance use [6], [18], [19], [20], as well as motor vehicle accidents related to drowsy driving [21], [22] and sport-related injuries [23].
Sleep restriction also contributes to several cognitive and behavioral problems that adversely impact academic performance and functioning. For instance, adolescents who are chronically sleep restricted perform academically poorer in morning classes and in overall performance [24], have increased absenteeism and tardiness [25], and a decreased ability to learn and retain material, actively participate in class, and perform decision-making tasks [7], [26], [27]. Furthermore, sleep compromised adolescents are also more likely to be depressed, anxious, irritable, defiant, apathetic, and impulsive than adolescents who achieve optimal sleep [28], [29], [30], [31].
To address the public health issue of chronic sleep restriction among adolescents, schools have made efforts over the past 15 y to change start times to occur later in the morning to better align with adolescents' circadian timing, social and environmental pressures, and to improve academic performance. Indeed, numerous cross-sectional and observational studies have suggested the benefit of delayed school start times. These studies usually compared one school district or class with another that used a later start time and revealed that adolescents in schools with later start times have less daytime sleepiness and sleep restriction [8], [32], [33], [34], [35], improved sleep quality [36], better behavior, attention and concentration in class [34], [35], less tardiness [37], higher academic achievement in some [35], [38] but not all schools [39], and fewer motor vehicle accidents [21]. The effects of later school start times compared favorably with other educational interventions in terms of overall cost and commensurate academic success [40].
While these studies suggest improvements relative to other schools or classrooms, interpretation of these findings is limited because they do not include assessment of within-subjects changes in sleep or other outcomes. Further, given the public advocacy for delayed school start times [41], the recent policy statement from the American Academy of Pediatrics to delay the start of class to 8:30 h or later [42], and some estimates that over 80 U S. school districts have already adopted later school start times [43], a review of the experimental literature is needed. Although others have written reviews with a similar focus, one was published in a journal that was not peer-reviewed [44], and a second consisted of a narrative review [45], a method that is not systematic in terms of the literature search or appraisal of quantitative data. Further, the American Academy of Pediatrics policy statement has called for research to document the effects of changes in school start times over time [42]. Thus, the purpose of this paper is to systematically review the evidence on the impact of delayed school start time interventions on students' sleep, health, and academic outcomes.
Section snippets
Literature search
We conducted a systematic review to synthesize the results of experimental research. The review began with a search of the literature to locate articles that used quantitative methods to assess the impact of delayed school start time interventions among youth. Relevant sources were identified by the first author through searches of the following electronic bibliographic databases in May 2014 with the assistance of a medical librarian to ensure balance of sensitivity and specificity: Ovid
Literature search
A total of 1,035 articles were identified and imported into Endnote software (Fig. 1). Duplicates were removed via the Endnote duplicate function and any remaining duplicates were manually removed, leaving a total of 818 articles. A thorough review of all article titles and abstracts was conducted to identify articles to review in full-text (N = 24). The majority of articles excluded after review of their title or abstract was due to a cross-sectional design or not related to a delayed start
Discussion
To our knowledge, this is the first systematic review conducted to evaluate and examine the effect of delayed school start times on sleep, health and academic parameters. Our review of primary experimental studies published in peer-reviewed journals provides initial, cumulative evidence that delaying school start times can be an effective method to improve important sleep outcomes, such as total sleep time and reduce daytime sleepiness, health outcomes, including reduced depression and caffeine
Funding source
KM was funded by a pre-doctoral fellowship from the National Institute of Diabetes and Digestive and Kidney Disease/NIH (T32-DK07718). NSR was supported by a grant from the National Institute of Nursing Research/NIH (P20-NR014126).
Conflict of interest
The authors have declared no conflicts of interest.
Contributor statements
Mr Minges conceptualized and designed the study, carried out the initial analyses, drafted the manuscript, and approved the final manuscript as submitted.
Dr Redeker contributed to the conception of the study and interpretation of data, critically reviewed and revised the manuscript, and approved the final manuscript as submitted.
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