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Circadian RhythmsCircadian 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. Circadian, derived from the Latin for "about a day," refers to the light-dark timing cycles that were first studied by a French astronomer in the 1700s, who observed how a plant behaved in a dark room. Subsequent research has shown that circadian rhythms influence the actions of plants, animals, and even single-cell organisms. Disruptions to our circadian rhythms from artificial light, erratic sleep and eating schedules, and other factors can cause many health issues. Circadian disruptions are linked to cardiovascular, gastrointestinal, sleep, and mental health disorders. Ongoing research seeks to better understand these relationships and unravel how to nudge our clocks back into sync.Explore Circadian Rhythms

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Circadian rhythms shift as we age—making older adults want to go to bed earlierThroughout our lives our internal biological clocks shift. Early-to-rise toddlers become hard-to-wake teens and eventually early-to-sleep seniors. Circadian rhythm fluctuations throughout our life cycle explain some of these changes. UCLA Health Temperature and certain medications can also affect your circadian rhythmsAlthough light and dark have the largest influence on circadian rhythms, temperature, stress, and certain medications are among the factors that also play a role. Persistent trouble sleeping or staying asleep may signal it’s time to see your doctor to get a better sleep-wake schedule. Merck Manual Parasites have circadian rhythms, tooResearchers 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. STAT News In 2017, researchers won a Nobel Prize for their work controlling fruit fly circadian rhythmsThis 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. Scientific American The body's sleep-wake cycle is one of its many circadian rhythmsThe body's sleep-wake cycle is one of its many circadian rhythms, which are behaviors or biological processes that exhibit 24-hour patterns. These rhythms are deeply impacted by light exposure, food availability, and much more, meaning where you live impacts your sleep. alie ward Research shows that, on average, humans correctly spot liars about half of the timePolygraph tests, also known as lie detectors, are better at detecting lies than people, but they still aren't allowed in most courtrooms. That's in part because of mixed signals. For example, spikes in heart rate can be explained by the stress a person feels during interrogation rather than by their own deception. As a result, polygraph tests are prone to false accusations. Results are also unreliable when participants actually believe their lies, examiners have biases, and tricks are employed to help test-takers relax. Howtown Artificial lighting and modern practices like working late are linked with sleep shiftsAn analysis of five adults participating in a German reality program that tasked them with living in a Stone-Age-like settlement for 2.5 months, provided some insights into how modern conditions may have shifted our circadian rhythms. The participants slept about 90 minutes more under these conditions, and went to bed earlier and rose earlier, raising questions about how circadian rhythms, sleep, and sleep quality are affected by modern lifestyles. Journal of Clinical Sleep Medicine Babies don't develop a circadian rhythm right awayNewborns don't develop circadian rhythm until they are a few months old, which helps to explain their erratic sleep schedules. When they're around three months old they begin to release melatonin, which influences sleep, and around the same time they develop more regular sleeping routines. Cleveland Clinic Most things you would get tested for at the doctor’s office are influenced by circadian rhythmIn this video, a Salk Institute Researcher offers daily habit tips focused on aligning with your circadian rhythms. She notes heart rate and blood pressure are highest in the afternoon, for example, and there’s a triglyceride peak in the morning. Generally, she says, it’s a good idea to schedule your doctor’s checkups at the same time of day each year so there’s consistency across your data. TEDx Talks Daylight Saving Time is linked to a rise in heart attacks and strokesLosing an hour of sleep with seasonal time changes is not good for our health and it’s linked to risks for the first couple days of Daylight Saving Time, including a short-term rise in heart attacks and strokes. Scientists theorize these elevated risks are linked to the disruption to the body’s circadian rhythm. American Heart Association Emulating Earth’s light-dark cycle is important for astronaut health and safetyAstronauts 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. NASA Circadian rhythm disruption can influence various mental health disordersThis 2020 research overview surveys the relationship between disrupted circadian clocks and disorders including schizophrenia, anxiety, and bipolar disorder. It incorporates findings and therapeutic approaches drawing from human night shift studies and jet lag studies alongside animal research. Nature There is a best way to wake up—but not a best timeSleep and circadian rhythm researchers say that waking up at a consistent time each day—even on weekends—and viewing morning sunlight are the best ways to keep body functions running at their peak. (Some users may encounter a paywall.) The New York Times SAD disrupts melatonin secretion, producing it longer in the morningThe hormone is secreted from the pineal gland in the brain and is key to the timing of sleep. However, the substance does not generate sleep itself. With seasonal affective disorder, the body misperceives the photoperiod—the time with light—and the cycle of sleeping and waking gets off-phase, leading to issues with circadian rhythms. TED-Ed Read Norman Rosenthal’s original 1984 paper describing SADThe study helped formalize research and diagnosis of seasonal affective disorder, catalyzing the development of therapies (like bright light) and leading to broader adoption of effective treatments. It also helped connect the influence of light and seasons on biological mechanisms in the human body, spurring exploration of circadian rhythms. National Library of Medicine Ending daylight saving time may prevent as many as 300,000 strokes annuallyChanging clocks twice a year disrupts circadian rhythms, which last an average of 24 hours and 12 minutes. Studies have shown that permanent daylight saving time provides fewer health benefits than standard time. Popular Science Astronauts follow strict schedules to mimic Earth days and stay healthy in spaceExperiencing 15 sunrises and sunsets a day while aboard the International Space Station, visitors preserve their circadian rhythms and mental health with fixed timetables. Astronauts have described sleep as living in a vacuum cleaner due to ongoing air filtration. European Space Agency Chronobiologists study the effect of time on biological cyclesCircadian 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. Quanta Studies suggest when you sleep is as important as how much you get"Molecular clocks" made of timekeeping proteins and genes help regulate the body and its processes, meaning the disruption of one cog will impact another. Those who live in sync with their circadian rhythms report less fatigue, clearer thoughts, greater medication benefits, and better long-term health outcomes. Reuters

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