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.
Related terms
- Melatonin
- Hormone the brain releases at night to signal sleep; also a supplement used to shift sleep timing.
- Sleep latency
- How long it takes to fall asleep after lights out — typically 10–20 minutes in healthy sleepers.