Effects of one night's sleep loss on nocturnal sleep, performance, and sleepiness were evaluated in 12 subjects (8 boys, 4 girls) whose ages ranged from 11.7 to 14.6 years. The magnitude and direction of sleep stage changes after sleep loss were comparable to similar findings in older subjects. Performance test decrements occurred for two measures during sleep deprivation. The performance decrements appeared to be related to episodes of sleep during the performance tests. Subjective measures of sleepiness showed a significant increase during sleep loss, with a complete recovery to basal levels after one night of sleep. The subjective ratings of sleepiness during sleep loss also showed a marked short-term dependence on preceding activity levels. Multiple sleep latency tests showed a marked reduction of sleep onset latency from 0530 throughout the day of sleep loss. In contrast to the subjective measures, sleep latency test scores did not vary with activity levels during sleep loss and did not recover to basal levels until the afternoon of the first recovery day. In general, there were no marked differences in the sleep loss response of young adolescents as compared to published reports of sleep loss in older subjects.