Control of sleep states in the rat by short light-dark cycles

The effects of 60-60 min light-dark (LD) cycles on sleep, feeding, drinking and motor activity were investigated in the rat. Waking, consummatory behaviour and motor activity were inhibited by light, while sleep was enhanced. The effects were dependent on the intensity of light. The percentage of slow wave sleep (SS) increased within the first 15 min of the light period and reached a plateau within 30 min. The reduction of SS during darkness showed a similar time-course. The percentage of paradoxical sleep (PS) started to increase only 15 min after the onset of light and continued to rise during the first 15 min of darkness. All parameters exhibited a free-running circadian rhythm. The LD-induced changes of SS were largest during the circadian phase in which the animal was active. The changes of PS which succeeded the onset or offset of light, were largest in the inactive circadian phase. The most immediate influence of changes in illumination is exerted on the slow wave component of sleep. Hypotheses to account for the temporary dissociation between the two sleep states are discussed.