REM sleep inhibition by light in the albino rat

Two groups of rats were maintained on a 12 hr light-12 hr darkness (L12-D12) schedule. One was subjected to continuous light and the other to continuous darkness for 3 weeks and then returned to the original schedule. Rats placed in continuous light, initially showed a marked decrement in both rapid eye movement sleep (REMS) and slow wave sleep (SWS) time. A dissociation developed later between the sleep stages; REMS time remained below baseline but SWS time increased above it. In recovery (L12-D12), a temporary increase of REMS time above baseline was observed. During the first 2 weeks of continuous darkness REMS time increased 30% and REMS/total sleep (%-REMS) increased 17%. Both measures returned to baseline during the third week of darkness. In recovery (L12-D12), REMS decreased 37% and %-REMS decreased 18%. Changes in SWS time paralleled those in REMS time in direction but were proportionately smaller relative to baseline values. It is proposed that light stimulation inhibits REMS in the rat whereas darkness is the preferred and, whenever possible, the utilized condition for elaboration of REMS. Rats housed under long-term exposure to L12-D12 schedules develop and maintain a chronic REMS debt though daily REMS values return to normal. Prolonged darkness seems to be necessary for a complete REMS rebound. The possibility that light inhibits SWS as well as REMS is also considered. The effects of the lighting environment on the stages of sleep are interpreted within the larger context of the rat's life style.