BACKGROUND Afternoon nap sleep was studied in 32 young male medical students who take customary naps to replace loss in nocturnal sleep. METHODS From 16 subjects, a group called dreamers was formed, and the other 16 individuals were grouped as non-dreamers. Polygraphic recordings lasting 30 min were done at a fixed time in the afternoon, and the relationship between these data and the occurrence of dreams was investigated. RESULTS We found that this replacing of nap sleep can adopt different sequences and relative durations of its phases, and can also show individual variations that have a systematic relationship with the occurrence of dreams. It was observed that dreaming was closely related to the appearance, during the first 10 minutes of the nap, of Stage I with Slow Eye Movements, interrupted by Sleep Onset REM Periods (SOREMPs) and, to a lesser degree, to phases IV and III of slow sleep. CONCLUSIONS According to these findings, the existence of dreamers and non-dreamers depends upon the relationship between an internal sleep-waking rhythm, and an external rhythm imposed by the daytime resting-activity schedule on the habit of dreaming, and, to a certain extent, on the mental phenomena occurring between the generation of dreams and the moment of awakening.