Circadian rhythms are internal biological clocks that function on roughly 24-hour cycles, prompting daily processes and behaviors on a cellular level. Circadian rhythms play a role in everything from our wound healing and digestion to how we sleep and metabolize drugs. Directed by a master circadian clock called the suprachiasmatic nucleus located in the hypothalamus, this central timekeeper coordinates the many peripheral molecular clocks located throughout the body. The clocks’ actions are driven by cues—formally called "zeitgebers"—that include light, food, temperature, medication, and exercise.
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The suprachiasmatic nucleus, the master clock in mammals, has 20,000 neurons that help maintain the light-dark cycle in the body. Among its jobs: It receives light input information from the retina and then signals peripheral body clocks to act on this information, driving everything from body temperature to hunger.
German for "time giver" or "time cues," common external zeitgebers include sunlight, food, and physical activity. The timing of these external influences can shift circadian clocks or lead to irregularities: A 2018 study, for example, found that later light exposure and meals were linked to later timing of peak activity and bedtimes.
Circadian rhythms were first observed in the 1700s, when French astronomer Jean-Jacques D'Ortous de Mairan put a plant in a dark room and watched it still respond to day and night cycles, setting off further studies about specific light-dark biological cycles, an area of inquiry that continues today with the human "clock proteins" at the molecular level.
This detailed circadian rhythm infographic breaks down a day in the biological life of a fly, providing a peek at some of the groundbreaking work that elicited a Nobel Prize in Physiology or Medicine.
Skin cells responsible for healing wounds work more quickly during the daytime, underscoring that the timing of an injury greatly influences recovery time, according to 2017 research. Nighttime burns took an average of 11 days longer to heal than burns incurred during the day. (Some readers may experience a paywall.)
Astronauts aboard the International Space Station experience 16 daily sunrises, but scientists have learned to try to promote circadian rhythms by phasing through several light settings aboard the ISS to improve astronauts’ sleep and productivity.
For reasons that are still not entirely clear, on average, people tend to lift more weight and achieve other exercise feats later in the day. Theories to explain this discrepancy include muscle clocks are at peak performance in the afternoon and that there’s morning-to-afternoon variation in core body temperature, hormones, and neural activity.
Researchers are working to unravel how circadian rhythms of parasites and their hosts affect one another. They’re investigating how to potentially target and disrupt those clocks for disease treatments.
Sunlight typically tells our brain to stop producing melatonin, a hormone that makes us sleepy, but in dark Arctic winters humans must adapt to life without much sunlight. Scientists in this video provide tips that include trying to increase the amount of light around you, even if it’s just table lamps.
When heart attacks occur during the day they typically do more harm to the heart tissue than if they occur at night. A 2025 study suggests that daytime heart attacks have higher levels of immune cells called neutrophils to contend with and they appear to worsen the injury. Scientists confirmed this same pattern in experiments with mice. (Some readers may experience a paywall.)
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