The female rat's sleep during oestrous cycle

The sleep-waking pattern of the isolated female rat showed a clear ultra-, circa- and infradian vigilance rhythm. The ultradian period was about 4-6 h. During the day, the amount of slow wave sleep (SWS) decreased and that of paradoxical sleep (PS) increased. In general, desynchronisation of the EEG increased during the day. The circadian period was determined by the light-dark cycle. Although the rats slept mainly during their inactive light phase, there was a considerable amount of sleep at night. The infradian period was determined by the individual length of the oestrous cycle. During the oestrous night more motor activity and less sleep was seen. During the following day a strong rebound of waking, SWS and PS occurred, but not of the 'connecting' sleep phases dozing and PS spindles. During the oestrous night the strong stimulation of the rat masked the ultradian rhythm. All oestrous effects disappeared in castrated female rats, which, compared to intact rats, showed more dozing during the night and the day as well as more PS during the hours of darkness.