In many animal species including humans, numerous processes exhibit 24-hour (h) rhythms. The circadian clock regulates daily rhythms of behavior and physiology such as the sleep-wake cycle (activity/rest), autonomic nervous function, and neuroendocrine function. The mammalian master clock located in the suprachiasmatic nuclei (SCN) of the hypothalamus incorporates environmental information and orchestrates peripheral clocks in other tissues and organs. Various characteristics of daily rhythms undergo age-dependent changes with respect to amplitude, entrained phase, free-running period (τ), and reentrainability. The mechanisms underlying aging of the circadian clock have not been fully understood. This review discusses current findings on age-related changes in daily rhythms of behavior and physiology
