Higher rates of car crashes among teen drivers seem to be linked with earlier school start times, according to a new study.

In 2008, the weekday crash rate for 16- to 18-year-olds was about 41 percent higher in Virginia Beach, Va., where high school classes began at 7:20 a.m. to 7:25 a.m., compared with adjacent Chesapeake, Va., where classes started at 8:40 a.m. to 8:45 a.m., the study said.

There were 65.8 car crashes for every 1,000 teen drivers in Virginia Beach, and 46.6 crashes for every 1,000 teen drivers in Chesapeake, according to the study.

In a secondary analysis that evaluated only the traditional school months of September 2007 through June 2008, the weekday crash rate for teen drivers was 25 percent higher in Virginia Beach than in Chesapeake, the study said. An investigation of traffic congestion in the neighboring cities did not reveal differences that might account for the findings.

"We were concerned that Virginia Beach teens might be sleep restricted due to their early rise times and that this could eventuate in an increased crash rate," study researcher Dr. Robert Vorona, associate professor of internal medicine at Eastern Virginia Medical School in Norfolk, Va., said in a statement. "The study supported our hypothesis, but it is important to note that this study does not prove cause and effect."

The study was published today (April 15) in the Journal of Clinical Sleep Medicine.

Crash peaks

Vorona's study involved data provided by the Virginia Department of Motor Vehicles. In Virginia Beach, there were 12,916 registered drivers between 16 and 18 years of age in 2008, and these teen drivers were involved in 850 crashes. In Chesapeake, there were 8,459 teen drivers and 394 car accidents, the study said.

The two adjoining cities have similar demographics, including racial composition and per-capita income, according to the study.

Further analysis by time of day found that, in the morning, the teen crash rates peaked when students would be commuting to school, from 7 a.m. to 7:59 a.m. for Virginia Beach and 8 a.m. to 8:59 a.m. for Chesapeake, the study said.

Teen crash rates were highest in the afternoon hours, from 2 p.m. to 6 p.m. in Virginia Beach, where schools dismissed at about 2 p.m., and from 4 p.m. to 7 p.m. in Chesapeake, where schools dismissed between 3 p.m. and 3:45 p.m., according to the study.

The importance of sleep

The average teen needs a little more than nine hours of sleep each night, according to the American Academy of Sleep Medicine. But during adolescence, a biological change shifts the typical onset of sleepiness to later at night. This delay can make it a challenge for teens to get enough sleep when they have to wake up early for school, researchers said.

Starting high school later in the morning may promote driver alertness by allowing teens to get more sleep at night , Vorona said.

"We believe that high schools should take a close look at having later start times to align with circadian rhythms in teens and to allow for longer sleep times," Vorona said. Insufficient sleep can also lead to mood disorders, academic problems and behavioral issues, he said.

Researchers are planning to do follow-up studies to see if there are any other potential ramifications of early high-school start times, Vorona said.

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