Nineteen children (8 girls, II boys) were evaluated in a total of 47 three-day sessions across three summers. Children were ranked according to Tanner's stages of secondary sexual characteristics. Nocturnal sleep was recorded from 2200 to 0800 hr each night. Multiple sleep latency tests were given at 2 hr intervals from 0930 each day. Nocturnal sleep time and REM sleep time remained constant across Tanner stages. Slow wave sleep time declined progressively across Tanner stages, with a 40% reduction from prepuberty to maturity. Daytime sleepiness was significantly greater in subjects at Tanner stages 3 and 4 than at Tanner stages 1 and 2. Subjects at Tanner stage 5 tended to be as sleepy as Tanner stage 3 and 4 subjects but did not differ significantly from the less mature subjects. No gender differences were found in daytime sleepiness for children at similar Tanner stages. More mature children were significantly sleepier at 1330 and 1530 than in the late afternoon and evening.