night progressed into a zone that is the equivalent of two nights without any sleep, resulting in massive debilitation.

Dinges told the workshop participants that his research shows a significant change in the learning curve associated with sleep loss. With 8 hours of sleep a night, subjects get better and better every day at the assigned task. With 6 hours of sleep, the learning curve is gone, and with 4 hours of sleep the negative impact on learning is even more apparent. Dinges noted that these data show that learning itself —that is, the ability to acquire information, retain it, and then use it repeatedly—is altered by sleep restriction. What this research does not show is the individual's subjective state. Even though young people may say they are tired, they can't tell how impaired they are. They may feel adapted to being tired, but performance tests show the opposite. In tests of sleepy subjects at a computer, researchers observed full 30-second lapses in which the computer alarm went off because the subject hadn't responded for a half minute. With 18,000 opportunities to monitor, no subject sleeping 8 hours had a single lapse. With 6 hours of sleep, a quarter of the subjects had a total of 37 lapses, which started to occur on day 7 and peaked on day 14. Nearly half the subjects with 4 hours of sleep had a total of 188 lapses. The first one occurred on day 6, and they peaked on day 13, Dinges reported.

Adolescents who are allowed to sleep in on the weekends may have an opportunity to pay back some of their sleep debt. While this may mean somewhat less sleep on Sunday night, in the view of some participants it is better than no repayment of the sleep debt.

Emotional Response

Adolescence is also a period of risk for emotional and behavioral disorders. Arousal, stress, or distress may interfere with sleep, setting up a vicious cycle in which emotions cause lack of sleep and lack of sleep exacerbates emotions. These emotions are also related to areas of the prefrontal cortex. Dahl discussed long-term research by his group into sleep, neuroendocrine, and biological measures in adolescents who have severe depressive disorders (Dahl et al., 1996). That work indicates that the most significant biological dysregulation appears to emerge during puberty and is particularly prominent around sleep onset. Youngsters with these difficulties have trouble going to sleep, and REM sleep comes earlier in the night. The study measured their cortisol levels every 20 minutes for 24 hours after they were acclimated to the environment. Depressed adoles-

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