The REM-NREM sleep cycle: renewal process or periodically driven process?

The question of the serial dependence of successive REM-NREM sleep cycles was examined. The experiments were performed in two different settings: 309 sleep episodes of 11 healthy young sleepers (age range, 20-36 years) were recorded under entrained conditions in the sleep laboratory; 5 of these subjects also slept in an isolation unit (underground apartment) with free-running sleep-wake cycles for a total of 107 sleep episodes. The covariances between the first three REM-NREM cycles were computed using an intraindividual cross-night approach. Significant negative covariances were observed. This result confirmed the assumption of serial dependencies between successive REM-NREM cycles. These data agree with the features of a periodically driven process and are incompatible with the alternatively hypothesized renewal model. The periodically driven process is similar in concept to the basic rest-activity cycle

Untersuchungen zur Stabilität ultradianer Rhythmen beim Menschen. (= Studies on the stability of human ultradian rhythms (author's transl.))

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

Phase shift in the REM sleep rhythm

The periodic alternation between REM and NREM sleep was analyzed. Usually, sleep records of consecutive nights of a subject are regarded to be independent events. However, it may be that consecutive nights are realizations of a continuously ongoing rhythm. This was tested in the present study. The temporal patterns of REM and NREM sleep in sequences of about 30 consecutive nights for 3 subjects were analyzed. The results show that only the onset of the first REM sleep phase during any one night may be predicted from the sleep onset time, whereas a systematic phase shift between consecutive nights was observed in the later REM sleep phases. Thus, the onset of later REM sleep phases is better predicted by assuming a rhythm with stable period length which controls the appearance of REM sleep phases in successive nights. Under the experimental conditions the phase shift was between 5 and 10 min per 24 hrs for the 3 subjects. The result is accordance with Kleitman's basic rest activity cycle (BRAC) hypothesis