A 15-day simulated jet lag experiment was performed in a time isolation laboratory using 15 old women (79-91 years, mean 82.9 years) and 10 old men (71-86 years, mean 80.5 years). After five full days and nights on the subject's habitual routine, the waketime of night 6 was advanced by 6 h, truncating the sleep episode. All subsequent events occurred 6 h earlier than normal. Adjustment to the phase shift was evaluated by the circadian rhythm in rectal temperature (measured every minute), polysomnography of each sleep episode, and daily averages of mood, activation, and performance speed. While the circadian rhythm amplitude reduction consequent upon the phase shift showed a tendency to be longer lived in the old men, both old groups showed fairly rapid adjustment of the timing of their circadian temperature rhythm, comparable (if not better) to that observed earlier in a study of middle aged men. However, sleep disruption (and daytime sleepiness) appeared to be longer lived in the elderly, showing little of the recovery over time observed in younger subjects. Thus, there appeared to be some separation in the elderly of the sleep disruption and circadian rhythm disruption effects of jet lag.