The delayed sleep phase syndrome is characterized by difficulty in falling asleep at a socially acceptable time of night and an inability to be easily aroused in the morning. Most commonly encountered in adolescents, this condition can produce daytime sleepiness and poor school performance, and it can lead to behavioral problems. The clinical features of the syndrome are described in 22 adolescents. Nine subjects participated in a protocol of polysomnographic recordings to simulate habitual "weekday" and "weekend" sleep patterns. There was a significant increase in total sleep time (p less than 0.005) and REM sleep (p less than 0.001) during the "weekend" sleep period. A multiple sleep latency test was performed between the two nights to assess daytime sleep tendency. Daytime sleepiness was maximal in the morning, with a tendency for greater alertness as the day progressed. The reduced amount of REM sleep during the "weekdays" plus the tendency for sleepiness in the mornings may contribute to the behavioral and educational difficulties seen in these patients. Recognition of this syndrome enables a specific sleep schedule change to be made that effectively treats the problem.