The effects of a 20-min nap before post-lunch dip were studied. The subjects participated in nap and no-nap conditions at intervals of 1 week. Their electroencephalogram (EEG) recordings of relaxed wakefulness, mood, performance, and self-rating of performance level were measured every 20 min from 10.00 to 18.00 h. For nap condition, the subjects went to bed at 12.20 h and were awakened after they slept for 20 min. The nap improved subjective sleepiness and self-rating of task performance, and suppressed EEG alpha activity. The results suggest a positive effect of a 20-min nap upon the maintenance of daytime arousal level.