Delaying the school start time will only result in adolescents staying up later and worsening the developmental delay. Is this true?

As we progress from childhood into adolescence, changes occur in sleep regulation. Firstly, delayed patterns of melatonin secretions lead to a delay in the timing of circadian rhythms of sleep. This is seen as a shift from morning centricity to evening centricity. Secondly adolescents also undergo a change in homeostatic sleep regulation. There is a slowing of the rate of sleep pressure build-up during wake, which results in reduced sleep propensity (1). These changes lead to the pattern of later sleep onset and later morning awakenings typically seen in teenagers. The average adolescent has difficulty falling asleep before 2300 hours (2). However, what has not changed are the sleep requirements of these adolescents. The recommended required sleep duration remains a regular 8 to 10 hours for every 24-hour period as per American Academy of Sleep Medicine guidelines (3). This would translate into a real-world sleep onset of at or after 2300 hours and a morning awakening of 0700 hours to 0800 hours in order for an adolescent to obtain the recommended required amount of sleep. However, most traditional school start times do not take these adolescent circadian patterns into consideration. They often have early school start times of before 0830 hours. This leads to a reduction in sleep duration resulting in chronic sleep loss (4). As an example, using data analyzed by the Centre of Disease Control in the United States on 50370 high school students, Wheaton et al reported that 68.4% of adolescents slept an average of 7 hours or less on weeknights, while 23.2% slept an average of 8 hours, 6.0% slept an average of 9 hours and 2.4% slept 10 hours or more (5). 

Chronic reduction in sleep duration in adolescents has been reported to have several effects on adolescent development. It is associated with poor academic performance (6), mood disturbances and depression, suicide ideation (7), weight gain and obesity (8), cardiovascular disease like hypertension (9) (10) and increase road traffic accident risks (11). With all of these deleterious effects, it would be prudent to enable adolescents to achieve adequate sleep in order to promote the health and optimal development of these teenagers. However, adolescents often fail to obtain sufficient sleep due to a combination of late bedtimes and early morning awakenings. As described above, the biological changes seen in adolescents have a large part to play in their later bedtimes. Other external factors such as school homework, extra-curricular activities after school, employment after school (12), the late-night use of electronic and technology devices (13) and absence of parental-set bedtimes also do play a part (14). However, early morning awakenings are often due to a single overriding factor, which is to get to school early in order to meet early school start times (12). 

Delaying school start times has been proposed as a means to enable adolescents to obtain adequate sleep, and to better the academic performance, health and emotional well-being of teenagers. In 2014, the American Academy of Pediatrics proposed policy change encouraging schools to modify their school start times to accommodate the sleep needs to adolescents and to prevent chronic sleep loss (15). 

But is there evidence that this measure would work? In the statement above, “Delaying the school start time will only result in adolescents staying up later and worsening the developmental delay.”, the possibility that delaying school start times would not be an effective countermeasure to insufficient sleep is suggested. The statement also suggests the possible mechanism for this, primarily that delaying school start times would lead to later bedtimes and likely shorter sleep durations. It also suggests that this would then lead to a worsening of developmental delay in adolescents. In this essay we will look at the evidence regarding adolescent school start times, delayed sleep bedtimes and the effects on development. 

Quality of evidence

Before we look at the actual evidence, we will need to look at the quality of the literature and data that exists regarding the issue of delayed school start times. Early reports of the effects delayed school start time on sleep were mainly abstracts. Allen et al published 3 abstracts in 1989, 1992 and with Kowakski in 1995 (24), (25), (26). Wolfson et al (27) and Carskadon et al (28) also published on this matter in 1995. All these abstracts were published in the journal of Sleep Research.  Most other studies that looked at this subject matter fall into 2 categories. They can be classified into either cross-sectional studies or longitudinal studies. Cross-sectional studies are comparative studies that looked at adolescent outcomes such as sleep duration, academic performance and health at 2 or more schools with different start times. Different populations with differing demographics were studied within a specified time period. The weakness of a cross-sectional study is that a cause and effect relationship cannot be proven definitively and only associations between start times and outcome measures can be demonstrated. Why then do cross-sectional studies form the bulk of the studies on this matter? These studies are more commonly performed mainly for practical reasons. There are numerous administrative and logistical issues that would have to be addressed for school start times to be changed just for a study. Issues such as work schedules of school staff and faculty, transportation arrangements, parental schedules, lesson timetables and after school activity schedules would all need to be managed. Therefore, longitudinal studies are much fewer in number. Longitudinal studies would entail collecting study outcome data on adolescents before and after a change in school start times. They can study the same population of students before and after the change is implemented or they can study different populations of students of a corresponding grade before and after the change (an example would be within the same school, all secondary three students for a year with early start times, followed by all secondary three students for a year with a delayed start times). This methodology would be able to identify changes in outcome variables in individual adolescents before and after implementing a change in school start times. Longitudinal studies can provide better evidence of causation provided proper controls are adequately put in place.  

Having now an idea of the limitations of quality of evidence that exist on this matter, we shall now look at some of the studies available. 

Sleep Variables

Does delaying school start times delay bedtimes and reduce sleep duration? We will look at some the individual studies that reported sleep variables as studied outcomes. We will first look at longitudinal studies and summarise the main points of using both longitudinal and cross-sectional studies. 


Carskadon et al in 1998 studied 26 students in a longitudinal study for 2 years in a Rhode Island high school, USA. In their first year (grade 9), they started school at 0825 hours. In their second year (grade 10), they had an earlier start time and started school at 0720 hours. Sleep schedule data from sleep diaries and actigraphy was obtained. Polysomnography and multiple sleep latency tests (MSLT) were also performed. They found that earlier school start times was associated with earlier awakening times and shorter sleep durations and subjects did not sleep any earlier. Earlier school start times were associated with shorter REM latency. MSLT measured shorter sleep latencies and greater REM sleep indicating increased sleepiness (16). It is worth noting that although this was a longitudinal study, the change implemented in this instance was an earlier start time rather than a delayed start time, Nonetheless, the results are relevant. 

Another longitudinal study was published by Danner et al in 2008. They conducted a study in Kentucky on 9966 students in year 1 with a school start time of 0715 hours and 10,656 students in year 2 with a school start time of 0830 hours. They studied bedtimes, wake times, Epworth Sleepiness Scale (ESS) scores as well as motor vehicle crash rates in these adolescents. They reported that in year 2, students had longer average sleep durations, less weekend catch-up sleep, lower ESS scores and reduced motor vehicle crash rates. They concluded that delayed school start times increased sleep of adolescents and reduced the risk of motor vehicle accidents (17). 

O’Malley et al performed a longitudinal study in Connecticut from 2001-2004 and published their findings in 2008. They looked at 297 students attending Wilton high school in 2001 and 2002 with a school start time of 0735 hours and subsequently 997 students attending the same high school in 2004 after the implementation of a delayed school start time of 0835 hours (a 40-minute delay). They found that although a later start time led to similar bedtimes, there was a later rise time which resulted in longer weeknight sleep durations (an average of 456 minutes compared to 422 minutes). There was also less issues with daytime sleepiness (18).

Owens et al published a longitudinal study in 2010. They studied 201 adolescents from a High School in Rhode Island and compared bedtimes and rise times before and after implementing a delay of 30 minutes to school start times. They reported that after the delay was implemented, there was an average of 45 minutes increase in sleep duration, an average advance of bedtimes by 18 minutes and later rise times. There was an increase in the percentage of students achieving 8 or more hours of sleep from 16.4% to 54.7%. The number of students obtaining less than 7 hours of sleep reduced by 79.4% (19). There was greater sleep satisfaction and lower reported rates of daytime sleepiness, fatigue and depressed moods as well. 

In Israel in 2011, Lufi et al conducted a study using actigraphy and tests of attention and mathematical performance on 47 adolescents for 2 weeks, the first week with a delayed school start of 1 hour and in the second week, a return to normal school start times of 0730 hours. There was also a control group with no change in school times. They reported an average of 55 minutes increase in sleep duration as well as increased scores of attention (20).  

Borlase et al conducted a study in New Zealand which assessed 212 students in grades 9 and 12 in 1999 and 455 students in grades 9, 11 and 12 in 2008. The change in school start times was a delay in school start times from 0900 hours to 1030 hours for student in grade 12 and above. There was no change in start times for grades 9 to 11. This change was implemented in 2006. They looked at sleep habits, technology use and ESS scores amongst others. They reported that grade 12 students had longer sleep durations, earlier bedtimes and later rise times after the change (21). They also found that more technology use was associated with less sleep on school nights. 

Boergens et al performed a study on 197 boarding students and assessed them with the School Sleep Habits Survey before and after the implementation of a delayed school start from 0800 hours to 0825 hours. After the change, they found that there was an average of a 29-minute increase in sleep duration. Average rise times were later on school days with no change in school night bedtimes. There was an increase in the number of students achieving at least 8 hours of sleep from 18% to 44%. They also reported reduced rates of daytime sleepiness, caffeine use and depression scores. However, sleep duration reverted back to baseline levels after a return to the earlier start times (22). 

Wahlstrom et al in 2014 in his longitudinal component of a larger study, managed to collect pre-change data from 446 students in Jackson Hole, Wyoming with a school start time of 0735 hours. The school then implemented a delayed school start time of 0855 hours. Looking at sleep durations, they reported an average increase of sleep duration of 0.7 hours after the delayed school start (23). 

Wolfson et al looked at 15 students and assessed them through grades 9 to 10. During grade 10, an earlier school start time was implemented (from 0825 to 0720). They found that the earlier school start resulted in a reduction of sleep duration of 39 minutes with no change in bedtimes. (27). 

These longitudinal studies show that delaying school start times leads to an increase in sleep duration. Therefore, later school start times do not lead to chronic sleep loss. There are also cross-sectional studies that assess sleep duration with regards to school start times. Most of these studies also demonstrate a longer sleep duration in students with later start times as compared to those with earlier start times. (16) (18) (19) (22) (23) (24) (29) (28) (30) (31) (32) (33).

Only 2 cross-sectional studies (24) (29) did not show a significant change in sleep duration after implementing a later school start time. Allen et al looked at 61 students in a cross-sectional study with a difference in 30 minutes start times (0730 hours compared to 0800 hours). The authors did report a longer sleep duration in the later starting group, but this difference was not statistically significant (24). In the other study by Wolfson et al (29), a total of 3120 students were surveyed, the difference between sleep duration was also not considered significant. But as the start time difference between the groups was only 20 minutes (0710 hours as compared to 0730 hours), the effect might not be detectable. Therefore, there is no evidence that later school start times lead to reduced sleep durations, which was proposed by the question statement. 

We now move on the effect of delayed school start times on bedtimes and rise times. Most studies have found that delayed school start times led to students having later rise times (16) (18) (19) (22) (23) (24) (29) (28) (30) (31) (32) (33). This is not surprising as most students would use the extra time afforded to “sleep in” and wake up later. 

More pertinent to this discussion is the question: does a delay in school start times lead to a later bedtime? And if so, does this affect overall sleep duration? Studies looking at this variable have yielded mixed results. Majority of studies show no differences in school day bedtimes (16) (20) (22) (23) (24) (27) (28) (29) (31) (34). 2 studies have shown an unexpected result, which is an earlier bedtime after a delay in school start times (19) (21). However, we have found 5 studies that have observed a later bedtime in schools that have a delayed start time. We will now look at these studies which report a later bedtime.

In a cross-sectional study conducted in Switzerland, Perkinson-Gloor et al looked at 2373 students compared to 343 students who had a 20-minute delayed school start time. Although they did report a later weeknight bedtime in the group with the later school start time, they also reported a longer weeknight sleep duration, later weekday awakening times and reduced daytime sleepiness in this group (35).

Li et al conducted a comparative cross-sectional study in 2013 that looked at 525 students in 6 schools in China. 2 schools acted as controls with a start time of 0730 hours. 2 schools had their school start time delayed by 30-minutes to 0800 hours and 2 schools had their school start time delayed by 60-minutes to 0830 hours. The authors found that later school start times were associated with later bedtimes. However later school start times were also associated with longer sleep duration and less daytime sleepiness. The average sleep duration increased by 15.6 minutes in the students who started 30-minutes later and 22.8 minutes in the students that started 60-minutes later. The authors also found that a short sleep duration was associated with reduced academic achievement (30). 

Wolfson et al performed a cross-sectional study on 205 students in grade 7-8 in the New England school district. They compared early start time students (0715 hours) with later start time students (0837 hours) using the School Sleep Habit Survey. They found that the later start group had later bedtimes, but only in Autumn. Again, they also had later rise times (over 1 hour) and longer sleep durations of 50 minutes. They also had less daytime sleepiness in Autumn, higher grades as well as less sleep-wake behavioural problems (32). 

Zhang et al published results of a cross-sectional study in 2010 on 4470 sets of mother-father-child trios looking at the factors that influenced sleep/wake patterns of the children. They looked at factors such as school start times, parental sleep/wake patterns, socio-demographics and daytime activities. They found that out of all the factors studied, school start times had the strongest influence. 3 groups of start times were studied, a morning school start time of 0735 hours, a whole day start time of 0808 hours and an afternoon start time of 1254 hours. The morning school start time group had the shortest time-in-bed and the greatest number of daytime napping. The afternoon start time group had a later bedtime but a longer time-in-bed and longer sleep duration. One point to note in this study, is that the average age of the children assessed was 9.2 +/- 1.8 years, so not every subject met with the World Health Organisation’s definition of an adolescent, which is 10-19 years (33).

Paksarian et al published in 2015 a cross-sectional study conducted from 2001 – 2004 on 7216 sampled students aged 13-19 years that was nationally representative of students in the USA. The mean school start time was 0801 hours, but the range was from 0705 hours to 0922 hours. Later start times were associated with a later bedtime. The average was found to be a 10-minute later bedtime for every 30-minute delay in school start times. However, they also found that later start time beyond 0801 hours was associated with longer sleep durations (36).  

In the studies mentioned above, most studies report a later start time was associated with longer sleep durations, including the studies that reported later bedtimes. The only studies (24) (29) that have reported no difference in sleep duration and the possible reasons have been discussed earlier. Almost all studies employed the use of self-reported sleep data such as questionnaires or sleep diaries, but 2 studies used actigraphy. The results from these studies were also consistent with a longer sleep duration for students who had a later start time (16) (20). Therefore, there is no evidence that later start times reduce sleep duration or result in chronic sleep loss, which is the main sleep factor leading to impaired adolescent development.  

Could delayed school start times be associated with increased daytime sleepiness? Delayed school start times were also associated with reduced daytime sleepiness in the above-mentioned studies (17) (18) (19) (21) (23) (30) (31) (32) (35), as well as 1 other study by Epstein et al (37) in Israel. However, 2 studies did observe no difference in daytime sleepiness in relation to school start times (38) (39). However, when we look at these 2 studies, the application of delayed school start time was only for 1 day of the week in the Norwegian study (38) or for alternate weeks in the Croatian study (39), where the students spent one week in morning sessions followed by 1 week in afternoon sessions. There is no evidence that delayed start time leads to daytime sleepiness or impaired daytime functioning. 

Academic Performance 

As an outcome for adolescent development, academic performance has often been measured in relation to adequate sleep and school start times. The results in aspect appear mixed. However, several confounders make the assessment of academic performance in relation to school start times difficult. Firstly, there often is no standard assessment employed by various schools at different points in the academic year or across the grades. Secondly, grading varies greatly according to the subject assessed, the teacher’s preferences and the school’s standards. Thirdly, academic grades might not be able to vary greatly in students who are already performing very well, therefore an effect may not be statistically significant. Most studies report a positive effect on academic performance after delaying school start times. Some studies show no effect (22) or even a negative effect (39). Again, in this Croatian study (39), the issue of alternating morning and afternoon sessions is an issue to be taken into consideration. More research is needed.


Some of studies mentioned above measure depression scores in relation to school start times. Many of these studies show that students in later starting schools have reduced depression symptoms and depression scores (19) (22) (23) (31). The Norwegian study (38) showed no difference in depression scores with later school start times. This might again be due to the fact that the delay was only applied on 1 day of the week.  

Other outcomes

Implementing a later school start time also yield benefits in other areas. Reaction times improved with a reduction in road traffic accidents. Crash rates reduced by 16.5% following a 60-minute delay in school start times (17) (40). Later school start times were also associated with improved outcomes for weight loss interventions in adolescents (41). 


Our goal was to evaluate the statement “Delaying the school start time will only result in adolescents staying up later and worsening the developmental delay.” What the evidence has showed so far is that this statement has no basis. The question that all policy makers and educators have is whether adolescents would use the schedule adjustment of a later start to school to get more sleep, or whether they would use the opportunity of the extra time for more late-night activities, leading to later bedtimes and further insufficient sleep. The evidence described above provide proof that later school start times lead to longer sleep durations, primarily by delaying rise times. Most studies show that even relatively small delays in school start times lead to significant increases in sleep durations. Some adolescents might have later bedtimes but even they did not have reduced sleep durations. In fact, they had increased sleep durations and less daytime sleepiness. So rather than extending their waketime activities at night, adolescents chose to “sleep in” before school, in line with the biological changes to their sleep patterns. As a result, most adolescents gained an increase in their overall weekly sleep quantities after a delay to school start times.  This has also led to fewer depression symptoms. Although not all studies show a positive impact on academic performance with later start times, no study has found a negative effect of later start times on academic performance. Overall the effects of delayed school start times on adolescents are beneficial. 


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