Starting the school day earlier may lead to more car accidents involving teenagers, new research suggests.
The study, which looked at schools in two cities in Virginia with different start times, found an association between earlier classes and more crashes among sleep-deprived students.
"Teenagers need over nine hours sleep a night, and it looks like a large number of teens don't get sufficient sleep... part of that relates to the time that high schools begin," said study author Dr. Robert Vorona, an associate professor of internal medicine in the Division of Sleep Medicine at Eastern Virginia Medical School in Norfolk, Va.
"There are data that demonstrate that lack of sleep has negative consequences for teens," he said. "And some data show that younger drivers are more likely to have crashes when they have inadequate sleep."
The study compared crash rates in 2008 for high school students with widely varying school starting times in Virginia Beach and Chesapeake, two adjacent cities with similar demographics. Virginia Beach's classes started at 7:20 a.m.; Chesapeake's began at 8:40 a.m.
While the overall accident rate for all drivers was higher in Virginia Beach, the difference between teens in the two cities was stark, Vorona said. Chesapeake had 46.2 crashes for every 1,000 teen drivers, compared to 65.4 per 1,000 teen drivers in Virginia Beach -- a 41 percent difference.
Teens are "biologically programmed" to get sleepy and wake up later than adults, said Barbara Phillips, of the University of Kentucky College of Medicine, a professor with the school's division of pulmonary, critical care and sleep medicine. "They truly can't help it. They're just not going to get sleepy at 10 p.m., so it's hard for them to get the eight to 10 hours of sleep they need to get when they have to catch the 7:30 bus."
Phillips is co-author of a study that compared car crash rates and increased sleep for adolescent drivers in Lexington, Ky., when the school district instituted a later school day in 1998. Data were analyzed from the two years before and after the change.
The study found that when teens increased their sleep, crash rates declined 16.5 percent during a period when teen crash rates throughout the state increased by 7.8 percent.