THE EFFECT OF OLD-AGE ON THE FREE-RUNNING PERIOD OF CIRCADIAN-RHYTHMSIN RAT

The free-running period is regarded to be an exclusive feature of the endogenous circadian clock. Changes during aging in the free-running p eriod may therefore reflect age-related changes in the internal organi zation of this clock. However, the literature on alterations in the fr ee-running period in aging is not unequivocal. In the present study, w ith various confounding factors kept to a minimum, it was found that t he free-running periods for active wakefulness, body temperature, and drinking behavior were significantly shorter (by 12-17 min) in old tha n in young rats. In addition, it was found that the day-to-day stabili ty of the different sleep states was reduced in old rats, whereas that of the drinking rhythm was enhanced. Transient cycles were not observ ed, nor were there any age-related differences in daily totals of the various sleep-wake states. The amplitudes of the circadian rhythms of active wakefulness, quiet sleep, and temperature were reduced, whereas those of paradoxical sleep and quiet wakefulness remained unchanged.