Twenty-four healthy, young (21-35 years old) men with no complaints of daytime sleepiness, no habitual napping, and polysomnographically verified normal nocturnal sleep extended their time in bed (TIB) to 10 h for 6 consecutive nights to assess the effects of sleep extension on daytime sleepiness and performance. Twelve subjects had basal average daily sleep latencies of less than or equal to 6 min on the Multiple Sleep Latency Test and 12 had latencies of greater than or equal to 16 min before TIB was extended. The sleep extension improved daytime sleepiness differentially in the two groups. The degree of improvement was greater in the sleepy subjects than the alert subjects and the pattern of improvement differed between the groups. Sleepy subjects showed an immediate and uniform increase in alertness, while alert subjects did not show improvements until late in the extension. However, sleepy subjects never achieved the baseline level of sleepiness/alertness seen in the alert subjects.