Most teenagers are chronically sleep deprived. One strategy proposed to lengthen adolescent sleep is to delay secondary school start times. This would allow students to wake up later without shifting their bedtime, which is biologically determined by the circadian clock, resulting in a net increase in sleep. So far, there is no objective quantitative data showing that a single intervention such as delaying the school start time significantly increases daily sleep. The Seattle School District delayed the secondary school start time by nearly an hour. We carried out a pre- / post-research study and show that there was an increase in the daily median sleep duration of min, associated with a 4.5% increase in the median grades of the students and an improvement in attendance.
Adolescents typically have a preference to stay active until late in the evening and to wake up late in the morning. This timing of daily activity or “chronotype” is not only a consequence of a change in social life and the use of electronic devices that keep teenagers awake during the evening, but is also a result of changes in both the circadian and homeostatic regulation of sleep (
. During puberty, the adolescent circadian system naturally delays the onset of sleep to a later time. One reason for this is an apparent lengthening of the circadian period during the teenage years ( (1) ), which typically leads to a later onset of the biological night relative to the light-dark cycle (
(1) ). On the other hand, the homeostatic regulation of sleep, which increases sleep pressure with waking hours, is also modified in older adolescents. This allows them to stay awake longer, relative to younger adolescents, due to a decreased sleep pressure during wake periods ( (
). On the basis of these measurable changes in sleep regulation, adolescents find themselves caught between two competing yet equally important forces: their circadian and homeostatic regulation of sleep, which delays sleep onsets, and their social obligations, which impose early sleep offsets resulting in a net decrease in daily sleep. Most adolescents sleep less than the recommended daily sleep at this age (8 to hours) (
(6) ), and an intervention that has been proposed to increase sleep is delaying school start times (American Academy of Pediatrics, 1743364. Although some studies have used survey data to show that when teens are allowed to go to school later, they report longer sleep times, so far, there is no objectively recorded data indicating that delaying the school start time lengthens daily sleep in adolescent students.
Increasing daily sleep duration in adolescents is not only critical because of the clear adverse physical and mental health outcomes associated with chronic sleep deprivation but also because of the role that normal sleep plays in learning and memory consolidation (
. Any action that results in longer daily sleep duration should also result in better academic performance. The link between longer sleep and better school performance has been hard to establish in field studies; whether delayed secondary school start times result in better performance also remains to be determined.
(9) and Horne-Östberg Chronotype Questionnaires.
Figure 1A presents the wrist activity mean waveforms for students pooled from both schools during each year. During school days, a two-way analysis of variance (ANOVA) yielded an effect of time [F(143, 25,311)=224.8, P F(143, 25,311)=18.43, P F(143, 25,025)=161.5, P F(143, 25,025)=2.19, P (P) ( P=0. 18, effect size=0. 2012). In contrast to school days, nonschool days did not show any significant differences between years in any of these sleep parameters. The number of naps (counted after inspecting every actogram) students took was very similar between (616 total naps, 0.6 naps per student) and () 353 total naps, 0. 80 naps per student).