A recurring biological pattern or cycle in a living organism, such as your sleep-wake cycle or daily body temperature changes.
From bio- (life) + rhythm (a repeated pattern in time), combining Greek bios with rhythmos. The term was popularized in the 1970s when scientists studied circadian rhythms in detail.
Your body runs on about 24-hour cycles for sleep, hunger, and even mood—this is why jet lag is so brutal: you're fighting your own internal clock that evolved over millions of years!
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