Tom frequently drives over the curb and has picked up a couple of dented fenders when parking his car at the high school, and Mary, so nice when she goes out in the evenings with her boyfriend, can be downright disagreeable with him the next morning.

So Tom's just an inexperienced, not-too-skilled driver and Mary's a typically moody teenager, right?

Maybe. But something else could be at work in both cases: Sleep debt, or partial sleep deprivation, a problem affecting a large number of teens and, to a lesser extent, college students.

"In both adolescents and young adults, sleep deprivation is rampant," said Peter Franzen, an assistant professor of psychiatry with a doctorate in clinical psychology, at the University of Pittsburgh School of Medicine.

Most adolescents need 8.5 to 9.25 hours of sleep a night, experts say, and many just aren't getting it.

According to the 2006 Sleep in America Poll by the National Sleep Foundation, only 20 percent of adolescents get nine hours of sleep on school nights, and nearly half of them say they sleep fewer than eight hours. Twelfth-graders reported an average of 6.9 hours per school night. What with after-school extracurricular activities, part-time jobs, homework and the distractions of at-home computers, cell phones and iPods, there just isn't time.

"Sleep is one of the first things high school and college students sacrifice to meet academic demands and other goals," Dr. Franzen added. "This is sort of a public health problem. Kids fall asleep in school; they fall asleep driving cars. There are good statistics on kids having accidents."

To complicate matters, experts say, most adolescents aren't sleepy at a time of day that would allow them to get nine hours of sleep before heading to school for a 7 or 7:30 a.m. start. They can't be: at puberty, their circadian rhythms, or body clocks, change for a few years. Instead of getting sleepy in the evening like younger kids or adults, they get energized.

"There are two issues," said Dr. Daniel Shade, director of the Allegheny General Hospital Sleep Disorder Center. "They don't get enough sleep and when they need sleep is not when they get it. ... It's a double whammy."

A more natural sleep pattern would be from around 11 p.m. to 8 a.m.

Local experts cited studies that backed that up.

Dr. Shade pointed to a study that surveyed kids' sleep patterns from the turn of the 20th century until 1994. Kids in 1994, he said, got at least three fewer hours of sleep than those of 1900. Back then, he theorized, many people had no electricity and went to bed when it got dark.

Another study written about in the journal Pediatrics said teens lost as much as 120 minutes of sleep a night after returning to school from summer or holiday vacation, said Dr. Loreta Matheo, an assistant professor of pediatrics, adolescent medicine, in the University of Pittsburgh School of Medicine. She's also a physician at Duquesne University.

"They didn't go to bed earlier," Dr. Matheo said. "They just slept in. ... If you get up when your body is ready, that's your natural time."

To combat the problem, some school districts around the United States -- including schools in Minnesota, Kentucky, Virginia and Connecticut -- have tried pushing high school starting times back to 8 a.m. or later.

It's a tool a number of local districts, including Pittsburgh city schools, have considered but not yet implemented. Opponents have argued changes in transportation schedules would be too costly or force younger children to wait for buses in the dark. They also cite negative impact on high schoolers' sports and extracurricular schedules and after-work jobs.

But according to a report on research done at the University of Minnesota and a Jan. 14 op-ed piece in the New York Times, the results have outweighed the complications: Better attendance, somewhat higher grades and better scores on achievement tests, and decreased lateness, behavioral problems and dropout rates.

Still, such change comes slowly. This is, one must remember, a nation that believes the early bird catches the worm.

"In certain countries they have siesta times, break times," said Dr. Todd Wolynn, a pediatrician with the Pediatric Alliance who also serves as medical director of the UPMC Mercy Pediatric Sleep Disorder program. "In the Western culture we don't have that. It's work, work, work. Kids are in school from 7 to 3, then they have extracurricular activities until 5, then maybe a job until 8, homework until 10 and then bed at 11."

And even then going to bed may not mean going to sleep, not if they've got a bedroom full of electronic equipment. No wonder some parents have to literally drag their teen out of bed when the alarm sounds.

Still, because of the time of their teens' energy surge, parents often don't realize they are what Dr. Wolynn calls chronically sleep-deprived. They don't see the kids yawning behind the wheel as they drive to school or nodding off in the first couple classes of the day. They may brush off their kid's irritability as a natural part of the teen years.

The above behaviors, though, often are signs of sleep debt, the experts say. So are being accident-prone, forgetfulness, poor concentration, poor school performance, a slowing of the ability to do tasks, depression, obesity, headaches and insomnia and sleep disorders. The immune and cardiovascular systems also can be affected, Dr. Shade said.

"If you're a college kid on an all-nighter and you stay up for 24 hours, your cognitive functioning performance is equal to that of someone with a .10 blood alcohol level," he added. In Pennsylvania, the legal blood alcohol limit for driving is .08.

Recognizing the signs of sleep deprivation is only part of a parent's job. They also need to help their adolescents get enough sleep.

That's a tough one, says Dr. Matheo, mother of a teenage son.

"It's really a challenge," she said. "In the end, a young person has to realize the consequences of his action. If they don't like it, they'll change.

"It's difficult to change an adolescent unless the adolescent buys into it. Kids this age feel they are invincible. You have to ask them what would have to happen for you to change this behavior. If you failed a test? If your boyfriend got annoyed at your grouchiness? If you missed the bus? You're a B student. Maybe you could be a B+ student. ...

"They have to buy into the consequence, which has to be something they want to avoid. You can't just say, 'go to sleep.' "