By Robert Preidt
The finding contradicts the common belief that teens will simply go to bed later if they can sleep later in the morning.
In fact, the study found that even those who did stay up a bit later got more sleep than teens who had to get up for early morning classes.
When high schools started classes at 8:30 a.m. or later, teens were more likely to get the recommended amount of sleep. The findings showed that they spent an average of 46 more minutes in bed than did teens whose schools started classes between 7 a.m. and 7:30 a.m.
Data on the students' sleep habits came from daily online recordings made by 413 teens during a school year (September through May) and the summer months of June through August.
"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 the study's lead author, Orfeu Buxton. He is an associate professor of biobehavioral health at Penn State University.
Teens who had the earliest school start times did tend to go to bed earlier than those whose classes started at 8:30 a.m. or later, but they still didn't get the recommended amount of sleep, the study found. One theory is that anticipation of an early wake-up inhibited their sleep, Buxton said in a university news release.
The study was published in the December issue of the journal Sleep Health.