The human biological clock or "circadian system" serves the function of preparing the body and mind for restful sleep at some times of day and active wakefulness at others. The observed circadian rhythms result from three interacting processes: endogenous rhythm generation mechanisms, entrainment mechanisms to keep these rhythms "on track," and exogenous masking processes stemming from changes in environment and behavior. In both advanced age and extraterrestrial travel these processes, particularly the latter two, can be dramatically effected with a consequent disruption in sleep and daytime functioning. We are currently studying the age-related effects on these processes in healthy 80 year olds using two different protocols. The first involved 36 h of constant wakeful bedrest which "unmasked" the endogenous component of circadian rhythms in various physiological and psychological functions. This experiment revealed little difference between nine old men and four young men in the endogenous body temperature rhythm, but revealed quite marked differences between the age groups in subjective activation and objective performance rhythms. A similar pattern occurred in the second experiment, which was a phase shift study comparing five old women with eight middle-aged men in their response to an acute 6 h phase advance in routine. Despite rather short-lived age-related differences in circadian rhythms, there were dramatic differences between the age groups in measures of sleep, mood, activation, and performance efficiency lasting for a week or more after the phase shift.