To test the hypothesis that the rapid eye movements (REMs) of paradoxical sleep (PS) transmit information about environmental light: dark cycles, spontaneous locomotor patterns were studied in rats following surgical disconnection of the extraocular muscles and thus deprived only of the eye movement component of REM sleep. Entrainment of the circadian spontaneous activity cycle to the light: dark cycle was abolished after sectioning the extra-ocular muscles of rats, but not in sham-operated controls. Endogenous (free running) circadian patterns of spontaneous activity were preserved in the operated animals. The results are consistent with our electrophysiologic studies showing that REMs induce light modulated electrical potentials in brain regions that influence circadian rhythms of activity.