Circadian Rhythms

The body's master clock has 20,000 neurons

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.

'Zeitgebers' like light are external cues for the circadian clock

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.

Chronobiologists study the effect of time on biological cycles

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.

In 2017, researchers won a Nobel Prize for their work controlling fruit fly circadian rhythms

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 wounds and burns heal fastest if they occur during the day

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.)

Emulating Earth’s light-dark cycle is important for astronaut health and safety

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.

World records are often broken later in the day—and scientists wonder if muscle clocks explain the difference.

Parasites have circadian rhythms, too

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.

Lack of sunlight during Arctic winters can fuel a winter slump and a circadian disconnect.

Heart attacks are less harmful at night

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.)