It was investigated whether the REM-NREM (rapid eye movement-non-REM) sleep rhythm has a stable period during long-term observations. Sequences of 17 to 31 consecutive sleep records were analyzed for 6 test subjects and 1 patient. Period stability was confirmed for three experimental conditions: a) undisturbed night sleep, b) inversion of the sleep-waking cycle, c) absence of external timing mechanisms. The period of the ultradian REM sleep rhythm is no integral submultiple of 24 h, so that the remainder causes a daily drift in the REM sleep rhythm. It is assumed that the ultradian process is controlled endogenously. In contrast to the circadian rhythm the ultradian rhythm appears to be free-running under normal conditions. The stability of the ultradian period has been shown in long-term observations.