What is the circadian rhythm?
"Circadian" comes from the Latin circa (about) and dies (day)—"about a day." Without outside cues, the human body clock runs slightly longer than 24 hours, and morning light resets it to 24 hours each day. This syncing is called entrainment.
How the body clock is built
- Master clock: the suprachiasmatic nucleus (SCN) in the hypothalamus takes light signals from the retina and coordinates rhythms throughout the body.
- Peripheral clocks: nearly every cell—in the liver, muscle and fat—has clock genes that also respond to cues like mealtimes.
- Clock genes: the proteins CLOCK, BMAL1, PER and CRY switch one another on and off in a feedback loop. The scientists who uncovered this in fruit flies won the 2017 Nobel Prize in Physiology or Medicine.
Melatonin and sleep
As it gets dark, the pineal gland releases more melatonin, signaling "night" to the body. Bright light—especially strong light in the evening—can delay melatonin release. Sleep is explained by the two-process model (Borbély, 1982): sleep pressure builds the longer you're awake and combines with the circadian rhythm. That early-afternoon slump is part of the rhythm, too.
What happens when your circadian rhythm is disrupted?
Shift work, frequent jet lag, late-night eating and screen light push your body clock out of step with your actual schedule. Research suggests that, over time, this can hurt not just sleep but blood sugar control and weight, and links to metabolic syndrome are being studied. Getting morning sunlight, waking at a consistent time and dimming lights at night are the basics of keeping your rhythm on track. Disrupted sleep can also be a sign of obstructive sleep apnea.
This article is general health information and is not a substitute for diagnosis or treatment. If symptoms persist, see a doctor.
FAQ
- Does taking melatonin fix a disrupted body clock?
- Low-dose melatonin timed correctly can help shift the clock, for example with jet lag, but consistent light exposure and wake times matter most; ask a clinician before long-term use.
Sources reviewed: 2026-10-25
References
- Borbély, A. A. (1982). A two process model of sleep regulation. Human Neurobiology, 1(3), 195–204.
- Czeisler, C. A., et al. (1999). Stability, precision, and near-24-hour period of the human circadian pacemaker. Science, 284(5423), 2177–2181.
- The Nobel Assembly at Karolinska Institutet. (2017). The Nobel Prize in Physiology or Medicine 2017: Jeffrey C. Hall, Michael Rosbash, Michael W. Young.