HACKENSACK, N.J. — It's midnight. Do you know where your teenagers are?

They're up in their bedrooms, IM-ing their friends, surfing the Net, playing Halo and watching Letterman. Or maybe conjugating French verbs or reading about the Industrial Revolution.

Doing anything, that is, except what they ought to be: sleeping.

America's teens are chronically sleep-deprived, according to the National Sleep Foundation's annual poll. Only one in five gets the recommended nine hours of sleep on school nights, and by the time they reach their senior year of high school, many are missing out on 12 hours of sleep a week.

Just ask Max Zaydes. The 17-year-old high school senior stayed up until 4:40 a.m. last Thursday working on a science project, then got up at 7, missing his 7:05 gym class. "That was bad," he said. "Thursday was pretty rough."

"We talk about a sleep debt," says Dr. Terry Carbone, head of The Center for Sleep Medicine at The Valley Hospital in New Jersey, of the adolescents she treats. "I tell them, `You never repay a sleep debt!' And you don't."

The consequences of exhaustion are measured in school tardiness, missed homework assignments and nodding heads in class.

"I had a friend who laid her head down on the desk and fell asleep, and she didn't wake up when the bell rang," says Alicia Buksar, another high school senior. "The kids walked out and left her there. Finally the teacher went over and said, 'Excuse me, I think you need to go to your next class."'

Lack of sleep contributes to lower grades, depression and less participation in exercise.

Most teens miss sleep because of the normal changes of adolescence, compounded by distraction. Sleep specialists speak of circadian rhythms, or the body's internal clock. As children reach their teens, that clock shifts: They are wider awake later at night, and prone to sleep later in the morning. This "phase delay" makes it difficult to fall asleep before 11 p.m.

School schedules, on the other hand, are built around busing needs and plans for daylight sports events, among other things. Most teens must get up around 6:30 or 7 a.m. to make it to school on time.

One of the key problems is the "electronic playground" most teens have in their bedrooms. Computers, televisions, video games and telephones — nearly all have at least one.

The temptation is irresistible. Adolescents with four or more such items in their rooms were much more likely to get an insufficient amount of sleep at night and almost twice as likely to fall asleep in school, the poll found.

Carbone, of The Valley Hospital, concentrates on removing, or limiting use of, "the big three" — television, video games, and computers — an hour or 90 minutes before bedtime.

"As parents, we can get the computer or TVs out of their rooms," she says.

What surprises Carbone is how clueless parents are about their children's sleep habits. Overall, 90 percent of parents polled felt their teens were getting enough sleep at least a few nights a week.