Age-related changes in circadian sleep-wakefulness rhythms in male rats isolated from time cues.

Sleep-wakefulness rhythms were recorded for at least 5 consecutive days in young adult (n = 6; 5-8 months) and old (n = 4; 30-32 months) rats. During the continuous recordings the rats were maintained in isolated cabins under constant temperature and constant dim, red light. The period length and amplitude of the free-running circadian rhythms of wakefulness (W), slow wave sleep (SWS) and desynchronized sleep (DS) were estimated by means of a cosine curve fitting program. Age-related amplitude differences were absent under the constant recording conditions. The circadian period of the W- and DS-rhythms, however, was significantly shorter in the old rats. These results are consistent with the notion that circadian organization is changed in old age.