"I have to put in a few hours of homework, get to volleyball practice, and do some other things," she said.

This morning, as she heads to class on about seven hours sleep, Liza will join the 80 percent of sleep-deprived US teens who don't get the recommended nine hours of sleep a night. Another 25 percent of teenagers are falling asleep in class, according to a poll by the National Sleep Foundation, an independent nonprofit that studies sleep and sleep disorders. The poll also found that many are arriving late and others are driving while drowsy.

Nature vs. Alarm Clock

"In the competition between the natural tendency to stay up late and early school start times, a teen's sleep is what loses out," said Jodi A. Mindell of St. Joseph's University in Philadelphia.

School-age children and teenagers should get at least nine hours of sleep a day, according to the National Center on Sleep Disorders Research at the National Institutes of Health.

Liza illustrates the problems faced by teenagers who want to be active in extracurricular activities. Most of the activities are after school and require two to three nights a week of attendance.

Obesity, Diabetes, Heart Disease

Erin Duffley, who at 18 and also attends Oakwood High, illustrates another part of the problem: The older the teenager, the less sleep he or she is likely to get.

"I usually stay up till 1 am," she said, "I have homework, I talk with friends on the phone, and I have my MySpace account."

With less than six hours sleep a night, she said she depends on coffee to keep her awake and once every so often, a caffeine pill.

Without enough sleep, a person has trouble focusing and responding quickly, according to NIH. The agency said there is growing evidence linking lack of sleep with an increased risk of obesity, diabetes and heart disease.