Over-scheduled and exhausted teens are getting behind the wheel with serious sleep deficits -- often with disastrous results.

More than 100,000 car accidents every year are caused by fatigue, and at least half of those accidents involve drivers younger than 25, according to NBC's "Today" show, and pulling all-nighters isn't the only reason kids get too tired to drive.

Teenagers' natural circadian rhythms prevent them from falling asleep before 11 p.m., said experimental psychologist Thomas J. Balkin, Ph.D on the "Today" show segment.

Add to that the busy days of most kids -- early-morning classes, after-school jobs and late-night socializing with pals -- and you have a recipe for tragedy. The average teen needs at least eight-and-a-half hours of sleep to function, and most kids only get about six, said Balkin, who's also chairman of the National Sleep Foundation.

According to a national joint study conducted by Liberty Mutual and Students Against Drunk Driving, nearly 36 percent of 3,580 students in grades 10-12 often drive to school in the morning while drowsy.

So just what does this mean for drivers? Slower reflexes, impaired judgment and reduced awareness.

If you think that sounds a lot like driving drunk, you're right on the money. Balkin, who is also chief of the Department of Behavioral Biology at the Walter Reed Army Institute of Research, told "Today" that driving while tired is very similar to driving while intoxicated -- and that it should be taken just as seriously.

Most dangerous, he said, is something called "microsleep" -- when the brain falls asleep for just a few seconds, but the eyes remain open.

What can parents do to prevent sleepy driving? Treat it like drunk driving: Tell your kids to call for a ride if they're too tired to drive, or tell them to pull over and take a 20 minute nap and then drink a caffeinated beverage. The combination of the short rest and the caffeine should be enough to fend off fatigue.

Do your teens drive tired?