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Biological rhythms are cyclical patterns of physiological and psychological activity that occur over regular time periods. They are governed by internal biological clocks (endogenous pacemakers) and influenced by external environmental cues (exogenous zeitgebers). Understanding biological rhythms is essential for explaining the sleep-wake cycle, the effects of shift work and jet lag, and the relationship between disrupted rhythms and mental health.
Key Definition: A biological rhythm is a cyclical change in the body's physiological processes or behaviour that recurs at regular intervals, governed by endogenous pacemakers and entrained by exogenous zeitgebers.
There are three main categories of biological rhythm, classified by their cycle length:
| Type | Cycle Length | Examples |
|---|---|---|
| Circadian | Approximately 24 hours | Sleep-wake cycle, body temperature, cortisol secretion |
| Infradian | More than 24 hours (days, weeks, months) | Menstrual cycle (~28 days), seasonal affective disorder (annual) |
| Ultradian | Less than 24 hours | Sleep stages (~90-minute cycles), basic rest-activity cycle (BRAC) |
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