"Teens starting school at 8:30 a.m. or later were the only group with an average time in bed permitting eight hours of sleep, the minimum recommended by expert consensus," said lead author Orfeu Buxton, associate professor of biobehavioral health at Penn State. "Later school start times were associated with later wake times in our large, diverse sample."
‘Both anticipation and sleep debt can misalign teens' circadian clocks from expected early wake timing on school days, interfering with having consistent sleep.’
Buxton and colleagues report their findings Dec. 1 in Sleep Health, the Journal of the National Sleep Foundation, which devoted an entire special issue to the topic.
Teens with the earliest high school start times -- 7:00-7:29 a.m. -- obtained 46 minutes less time in bed on average compared with teens with high school start times at 8:30 a.m. or later.
School start times after 8:30 a.m. were associated with increased time in bed, extending morning sleep by 27-57 minutes compared to those teens with earlier school start times.
A common argument against later school start times is an assumption that teens will just stay up later.
"The presumption is if you let kids start school later they will simply go to sleep later and still not get enough sleep," Buxton said. "But that's a hypothetical scenario. There wasn't data to back that up."
While researchers did find that teens with the earliest school start times were going to bed earlier than those with 8:30 a.m. or later, the teens with earlier start times still did not get the recommended amount of sleep. Only those teens with schools that had a start time of 8:30 a.m. or later actually got the recommended amount of sleep, Buxton said.
One theory is that, despite going to bed earlier than their peers, teens with the earliest school start times didn't get enough sleep possibly due to anticipation of an early wake time the following morning, according to Buxton.
In addition, the investigators considered other research that looked at teens' "sleep debt," where teens make up for lost sleep on non-school days, leading them to wake up consistently and significantly later than those on school days.
Four hundred and thirteen teenagers completed an online daily diary each evening, beginning after 7 p.m., during seven consecutive days, including school days and non-school days during both the academic year and the summer, which was defined as September through May and June through August, respectively.
From each diary entry, researchers looked at the participants' reports of the previous night's bedtime, the time the teen woke up in the morning, whether or not the teen went to school, and the school start times.
Data collection included daily diary data from a subsample of the parent study, the Fragile Families and Child Wellbeing Study, which follows a longitudinal birth cohort of children born between 1998 and 2000 in 20 United States cities.