The suprachiasmatic nucleus (SCN) of the hypothalamus is considered to be a critical component of a neural oscillator system implicated in the timing of a wide variety of biological processes. The circadian cycles established by this biological clock occur throughout nature and have a period of approximately 24 h. With advancing age, however, these daily fluctuations deteriorate, leading to disrupted cycles with a reduced amplitude. In humans, age-related changes have been described for hormonal rhythms, body core temperature, sleep-wakefulness and several other behavioral cycles. It appears that the disruption of circadian rhythms and the increased incidence of disturbed sleep during aging are paralleled by age-related alterations in the neural and temporal organization of the SCN and a decreased photic input to the clock. The many lines of evidence of age-related decrements in circadian time-keeping and the observed neuronal degeneration of the SCN in senescence strongly suggest that the circadian pacemaker in the human brain becomes progressively disturbed during aging.