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Circadian Rhythms

The body's internal clock and the biology of daily cycles

Seed · early Updated Jul 19, 2026 3 min read
timebiologyhealth

The body runs on clocks. The main clock in the brain sits in the suprachiasmatic nucleus of the hypothalamus, a group of about 20,000 neurons. Clocks in organs such as the liver, muscle, heart, and pancreas keep local rhythms, while the brain clock helps align them with the light-dark cycle.1

The rhythm persists without daily time cues, but it does not land on exactly 24 hours. A controlled study that separated the body clock from the sleep schedule measured an average period of 24.18 hours in healthy adults.2 Light and other cues reset that drift each day.

Light synchronizes the clock to the environment. Melanopsin-containing retinal cells help set circadian timing and also contribute to visual processing.3 Short-wavelength light can produce a strong circadian response, but timing, brightness, duration, spectrum, and prior light exposure all matter.4

Shift work is linked with several health problems, including cardiovascular and metabolic disease, but much of the human evidence is observational.5 The International Agency for Research on Cancer classifies night-shift work as “probably carcinogenic to humans,” based on limited human evidence, sufficient animal evidence, and strong mechanistic evidence.6 That is a risk classification, not proof that a given schedule caused a given cancer.

Alertness changes across the day, but there is no one peak shared by every person and task. Chronotype, recent sleep, age, light, and the kind of work shift the pattern. Many people feel an early-afternoon dip even without a heavy lunch.

Sleep timing and duration both matter. A well-timed short night is not a sound replacement for enough sleep. The practical aim is regular sleep that fits both sleep need and circadian phase.

Electric light extends the day and lets people choose their light level. Bright evening light can delay circadian timing, while morning light can shift it earlier. A phone screen is only one part of the evening: content, habit, and lost sleep time can matter too.4

Start with the full pattern. Get outdoor light after waking. Dim the room before bed. Keep sleep and wake times as steady as life allows. If a screen keeps you awake, change its brightness, timing, content, or place in the routine. Blue-blocking glasses may help some people, but the evidence does not make them a cure for every sleep problem.

Go Deeper

Books

  • Why We Sleep by Matthew Walker — Neuroscientist on sleep’s role in health, learning, and creativity. Compelling but read critically (some claims overstated).
  • The Body Clock Guide to Better Health by Michael Smolensky & Lynne Lamberg — Practical applications of chronobiology.
  • Internal Time by Till Roenneberg — On chronotypes and social jet lag from a leading circadian researcher.

Essays

  • The 2017 Nobel Prize in Physiology or Medicine — The official press release traces Hall, Rosbash, and Young’s work on molecular clocks.7

Films

  • Sleepless in America (National Geographic, 2014) — Documentary on sleep deprivation’s effects on health, safety, and cognition.

Sources

  1. John H. Abel and Francis J. Doyle III, “A systems theoretic approach to analysis and control of mammalian circadian dynamics”, Chemical Engineering Research and Design 116 (2016), 48–60. ↩

  2. Charles A. Czeisler and colleagues, “Stability, Precision, and Near-24-Hour Period of the Human Circadian Pacemaker”, Science 284, no. 5423 (1999), 2177–2181. ↩

  3. Tiffany M. Schmidt and colleagues, “Melanopsin-positive intrinsically photosensitive retinal ganglion cells: from form to function”, The Journal of Neuroscience 31 (2011). ↩

  4. Christine Blume, Corrado Garbazza, and Manuel Spitschan, “Effects of light on human circadian rhythms, sleep and mood”, Somnologie 23 (2019), 147–156. ↩ ↩2

  5. Gloria Hulsegge and colleagues, “Shift work and health outcomes: an umbrella review of systematic reviews and meta-analyses”, Journal of Clinical Medicine 11, no. 3 (2022), 635. ↩

  6. International Agency for Research on Cancer, “IARC Monographs Volume 124: Night Shift Work”, 2020. ↩

  7. Nobel Prize, “The 2017 Nobel Prize in Physiology or Medicine: Press release”, 2 October 2017. ↩