The effect of napping for varying durations after one night of sleep deprivation was examined. Sleep latency tests were used to determine levels of sleepiness/alertness at 2, 4, 6, and 8 hrs following a morning nap of 0, 15, 30, 60, or 120 min duration. Ten normal-sleeping, young adult volunteers spent two consecutive days and the intervening night in the sleep laboratory on each of five weeks. Baseline sleep latencies were recorded the first day, sleep was deprived that night, a nap was taken at 0900 hrs, and sleep latencies were again recorded on the second day. The naps had differential alerting effects related to their duration, but none of the naps returned mean sleep latency for the 8 hrs to its basal levels. Alertness increased with nap duration, reaching its highest level with a 60-min nap; the 120-min nap was no more alerting than the 60-min nap. During the second hour of the 120-min nap, sleep became more fragmented with more shifts to stage 1 sleep or wake. Increased alertness was not strongly related to the sleep stage composition of the naps, the best predictor being minutes of slow wave sleep. Increased alertness was not detected until the second latency test 4 hrs after napping.