A study has been made of the effects of different photoperiods on paradoxical sleep (PS) in rats. The EEG recordings from olfactory bulb, hippocampus, and cerebral cortex were used to monitor slow-wave sleep, arousal, and PS. Triggering PS by a 5-min period of “lights-off” every 30 min during the day was confirmed. Rats were exposed to short days of 1, 4, and 12 hours composed of equal periods of light and darkness alternating continuously. During these abbreviated days, most of the PS occurred in the dark whereas under normal 24-hour diurnal lighting the bulk of PS occurs in the light. The photic induction of PS seems to be an unconditioned response specific to the lights-off stimulus. Attempts were made with lights-off as the unconditioned stimulus to condition PS to an auditory, conditioned stimulus. Pairing the tone and lights-off did not interfere with the photic induction of PS, but the tone alone was ineffective in triggering PS. Temporal conditioning did not occur. Finally, PS was not elicited by lights-on. Thus, lights-off appears to be a relatively specific, unconditioned stimulus for PS induction. The eyes are necessary for the effect to appear, but the pineal gland is not.