Teen drivers who start school earlier in the morning may be prone to more automobile accidents, according to a new U.S. study.

Students may not be so alert, the study suggests, since early school start times may promote sleep loss and daytime sleepiness.

Published in the Journal of Clinical Sleep Medicine, the study's lead author, Dr. Robert Vorona, said that starting high school later in the morning might make young drivers more alert because they get more sleep.

The study compared school start times and automobile crash rates for students aged 16 to 18 years in Virginia Beach, Va., where high school classes began between 7:20 a.m. and 7:25 a.m., to students at schools in adjacent Chesapeake, Va., where classes started between 8:40 a.m. and 8:45 a.m.

There were 65.8 automobile crashes for every 1,000 teen drivers in Virginia Beach, and 46.6 crashes for every 1,000 teen drivers in Chesapeake. The comparisons were made in 2008 and were similar to results in 2007.

"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," said Vorona who is an associate professor of internal medicine at Eastern Virginia Medical School. "Too many teens in this country obtain insufficient sleep. Increasingly, the literature suggests that this may lead to problematic consequences including mood disorders, academic difficulties and behavioral issues."

Another study in the April edition of the Journal of Clinical Sleep Medicine suggests that delaying school start times by one hour could improve attention levels and performance, and reduce mistakes. The Israeli study of 14-year-old, eighth-grade students found those teens who slept 55 minutes longer each night performed better on tests that require attention.

Vorona's study involved data provided by the Virginia Department of Motor Vehicles. It was supported by the Eastern Virginia School of Medicine Division of Sleep Medicine.