Biological rhythms govern vital bodily functions and are influenced by internal endogenous pacemakers and external exogenous zeitgebers. The three main types are circadian rhythms (24-hour cycles), ultradian rhythms (multiple cycles per day), and infradian rhythms (less than one cycle per day). Circadian rhythms regulate sleep-wake cycles and core body temperature. Research by Michel Siffre demonstrated the persistence of circadian rhythms even in isolation. Infradian rhythms include seasonal affective disorder and menstruation. Understanding these rhythms has important implications for health and behavior.
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