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The ~24-hour internal clock that governs sleep, alertness, and hormone release, set mainly by light.
The circadian rhythm is a roughly 24-hour internal clock, centered in the brain’s suprachiasmatic nucleus, that drives the timing of sleep, alertness, body temperature, and hormones such as melatonin and cortisol. Light is the dominant cue that sets it. Misalignment from shift work, jet lag, or late-night screens degrades sleep quality even when total sleep time looks adequate.