The effect of a 90-min nap period on mood was studied in 22 sleep-deprived patients with a diagnosis of major depression. All patients remained awake from 7 AM until 12 noon the following day at which time they were permitted to nap while being monitored by sleep encephalography. Fifteen subjects showed a significant response to sleep deprivation as defined by a 35% improvement on the Hamilton Rating Scale for Depression. After the nap a relapse of depressive symptoms occurred which was significantly related to the amount of non-rapid eye movement sleep time.