Many adolescents do not get enough sleep. Average nighttime sleep declines from more than ten hours at age 9 to slightly less than eight hours at age 16. Adolescents tend to be sleepy during the daytime even when they sleep a full nine hours, suggesting that they need as much as or more sleep than before. "Sleeping in" on weekends does not make up for the loss. In the WHO study, an average of 40 percent of adolescents (mostly boys) in 28 industrialized countries reported morning sleepiness at least once a week, and 22 percent said they are sleepy most days.

A pattern of late bedtimes and oversleeping in the mornings can contribute to insomnia, a problem that often begins in late childhood or adolescence. Daytime naps worsen the problem. In a longitudinal study of 2,259 middle school students, sixth-graders who slept less than their peers were more likely to show depressive symptoms and to have low self-esteem.

Why do adolescents stay up late? In part it may be because they need to do homework, want to talk on the phone with friends or surf the web, or wish to act "grown up". However, physiological changes also may be involved. Adolescents undergo a shift in the brain's natural sleep cycle, or circadian timing system. The timing of secretion of the hormone melatonin is a gauge of when the brain is ready for sleep. After puberty, this secretion takes place later at night. Thus, adolescents need to go to bed later and get up later than younger children; yet most secondary schools start earlier than elementary schools. Their schedules are out of sync with students' biological rhythms. Teenagers tend to be least alert and most stressed early in the morning. Starting school later, or at least offering difficult courses later in the day, would help improve students' concentration.

Many adolescents do not get enough sleep. Average nighttime sleep declines from more than ten hours at age 9 to slightly less than eight hours at age 16. Adolescents tend to be sleepy during the daytime even when they sleep a full nine hours, suggesting that they need as much as or more sleep than before. "Sleeping in" on weekends does not make up for the loss. In the WHO study, an average of 40 percent of adolescents (mostly boys) in 28 industrialized countries reported morning sleepiness at least once a week, and 22 percent said they are sleepy most days.

A pattern of late bedtimes and oversleeping in the mornings can contribute to insomnia, a problem that often begins in late childhood or adolescence. Daytime naps worsen the problem. In a longitudinal study of 2,259 middle school students, sixth-graders who slept less than their peers were more likely to show depressive symptoms and to have low self-esteem.

Why do adolescents stay up late? In part it may be because they need to do homework, want to talk on the phone with friends or surf the web, or wish to act "grown up". However, physiological changes also may be involved. Adolescents undergo a shift in the brain's natural sleep cycle, or circadian timing system. The timing of secretion of the hormone melatonin is a gauge of when the brain is ready for sleep. After puberty, this secretion takes place later at night. Thus, adolescents need to go to bed later and get up later than younger children; yet most secondary schools start earlier than elementary schools. Their schedules are out of sync with students' biological rhythms. Teenagers tend to be least alert and most stressed early in the morning. Starting school later, or at least offering difficult courses later in the day, would help improve students' concentration.

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