The aim of the present study was to assess the circadian variation of rapid eye movement density (REMD) in night sleep and daytime naps of young males. Daytime naps were scheduled in 12 healthy young males at 2-h intervals between 0800 and 2400 h. Each subject performed nine naps which were scheduled in randomized order across nine nonconsecutive days. A double-nap strategy was applied with 30 min of sleep in Nap A and one complete REM sleep episode in Nap B. Naps A and B were separated by a 10-min break. All night sleep recordings were performed for each subject at the beginning and the end of the series of naps. REM density was calculated for REM sleep episodes in Nap B and for each REM episode of the two nights. REM density was defined as the ratio of the number of 3-sec epochs of REM sleep with at least one rapid eye movement and the total number of all 3-sec epochs with REM sleep. REM density showed a steady decrease from morning to late evening. During night sleep, REM density increased from the first to the second and last third of the night. REM density paralleled the circadian distribution of tonic REM sleep parameters, which is roughly inverse to that of body temperature. These results show that not only tonic but also phasic parameters of REM sleep display a circadian distribution.