Circadian Rhythms

Circadian rhythms are endogenously generated daily cycles in physiology and behavior. In this context, the term endogenous means that circadian rhythms are dependent on internal biological timing mechanisms, referred to as circadian “clocks,” “pacemakers,” or “oscillators.” As implied by the combined circa and dian, these timing mechanisms produce biological rhythms with period lengths of approximately 24 hours. When exposed to the precise 24-hour periodicity of the environment—for example, to daily 24-hour light-dark cycles—endogenous circadian clocks become synchronized or entrained to those environmental cycles. In contrast, when maintained in experimental isolation from daily time-cues, circadian clocks express their endogenous “free-running” periodicity, which only approximates 24 hours. Researchers generally consider the free-running period as expressed in constant darkness to best reflect the period of the endogenous circadian clock.