He used to be my "morning child," the one who'd wake up sunny and raring to go in the morning. But somewhere around middle school he changed. The early-to-bed rules around our house were complied with, reluctantly, but he'd lie awake sometimes for hours, waiting for sleep to come. Weekday mornings became a loud, unpleasant time — for parent and child alike. What happened? Did the pod people do an exchange of children in the night? Come to think of it, he does look different and he certainly now speaks a language I sometimes have difficulty understanding.

The changes, though, seem to have another explanation. In addition to the remarkable physical changes so readily apparent in teens, adolescence brings with it other biological changes that impact on the body's internal "clock," delaying the natural time that teens fall asleep and awaken. Which would be fine if teens ran the world, but their natural inclination runs smack into the fact that many high schools in the United States start before 7:30 a.m.

In addition, they are faced with a world of evening stimulation — part-time jobs, homework, computers, televisions and electronic games in their rooms — that all conspire to delay bedtime. To make matters worse, teens require just as much sleep as they did in elementary school. According to Mary Carskadon, Ph.D., professor of psychiatry and human behavior at the Brown University School of Medicine and a leading sleep researcher, teens require on average more than nine hours of sleep each night.

Sleep and School Performance

In a survey of 3,000 high-school students in Rhode Island, Dr. Carskadon and her colleagues found that the teens on average were getting just 7.3 hours of sleep each night. Only 15 percent of them were receiving 8.5 hours of sleep each night, and a whopping 26 percent were sleeping only 6.5 hours or less. While Carskadon hesitates to equate increased sleep with better grades, since so many factors enter into student performance, she and her colleagues noticed that students who had mostly As on report cards received on average an hour more sleep each night and retired an hour earlier than students who had mostly Ds and Fs on their report cards.

"What it looks like in kids who are getting insufficient sleep is that they are waking up when their internal clock tells them they should be sleepiest," Carskadon says. Falling asleep in early classes is not uncommon. As the day progresses and the external signals — light, activity, even caffeine — kick in, the teens become more alert. "They feel like they're in the pits in the morning, then feel better later and they forget how crummy they felt earlier." By nighttime the cycle repeats itself.