Study objectives: Previous studies have demonstrated that albino but not pigmented rats show acute increases in REM sleep following light-to-dark transitions. Light and dark have also been shown to have direct effects on NREM sleep and wakefulness in albino rats. Little is known, however, about the direct light-dark effects on sleep patterns in pigmented animals. The purpose of the present study was to compare the direct effects of light and dark on REM sleep, NREM sleep, and waking in albino Lewis and pigmented Brown Norway (BN) rats.
Design: Groups of albino Lewis and pigmented Brown Norway (BN) rats were exposed to various light-dark (LD) schedules. In the first experiment, the lighting schedules were LD 12:12 and LD 3:3. The second experiment compared LD 12:12 with an irregular schedule consisting of short light and dark periods of unequal length.
Measurements and results: Both Lewis and BN rats slept more during the light and were awake more during the dark on all schedules. REM sleep patterns in light and dark periods were opposite, however. Lewis rats spent more of their sleep in REM sleep during dark than the light, whereas BN rats had a higher proportion of REM sleep in the light.
Conclusions: The results suggest that there are substantial direct effects of light and dark on sleep in pigmented as well as in albino rats, although these effects are not always the same in magnitude or even in direction.