Effects of light and circadian rhythm on the occurrence of REM sleep in the rat

Abstract

The factors controlling the occurrence of REM sleep (REMS) in the rat may be investigated on the level of the intrinsic REMS cycle (~10 min) or on the level of the circadian cycle (~24 hr). REMS is enhanced in the period that follows immediately after a light-dark (LD) transition. Although the mechanism or the biological significance of the phenomenon is unknown, LD-induced REMS may be useful for studying the REMS recovery cycle. Short LD cycles with a period of 10 min proved to be optimal for triggering REMS episodes. The circadian distribution of sleep is similar under entrained (LD 12:12) and free-running conditions (e.g., continuous darkness), and hence is mainly determined by endogenous factors. The predominance of slow wave sleep (SWS) in the beginning of the daily rest phase and the slight increasing gradient of REM throughout the phase render the circadian sleep pattern in rat and man rather similar. Episodes of non-REMS (NREMS) are long at the beginning of the rest phase and then become progressively shorter, whereas the duration of REMS episodes is more constant. On the other hand, the episode frequency of the two sleep states varies in parallel. The daily amount of REMS is enhanced during schedules with continuous or intermittent light. It is concluded that (1) the succession of NREMS and REMS within the REMS cycle reflects the interdependence of the two sleep states, and (2) the differences in their circadian distribution are principally a result of specific regulatory mechanisms determining the maintenance and 'intensity' of NREMS episodes.