cir·​ca·​di·​an sər-ˈkā-dē-ən How to pronounce circadian (audio)
: being, having, characterized by, or occurring in approximately 24-hour periods or cycles (as of biological activity or function)
circadian rhythms in activity

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In 1959, a scientist formed the word circadian from the Latin words circa ("about") and dies ("day"), and it caught on quickly. Most often, it's seen and heard in the term circadian rhythm, which refers to the inherent cycle of about 24 hours that appears to control various biological processes, such as sleep, wakefulness, and digestive activity. If you want to impress your friends, you can also use the term circadian dysrhythmia, a fancy synonym of jet lag.

Examples of circadian in a Sentence

Recent Examples on the Web Nap in the early afternoon, as that aligns with a decrease in energy levels after lunch and with the body’s natural circadian dip, which is an increase in sleepiness similar to what occurs at dusk. Steven Bender, Discover Magazine, 1 Sep. 2023 Disrupts Sleep Cycles Research has linked phone and screen use to disruptions in circadian rhythm, or natural sleep-wake cycle. Mark Gurarie, Health, 27 Aug. 2023 Our bodies already have clocks, with circadian rhythm designed to respond to light as a signal to wake. Sherry Walling, Fortune Well, 15 Aug. 2023 The idea behind blue-blocking lenses is to stop the light from entering the eye and throwing the circadian rhythm off; however, this has not been established with any degree of certainty in clinical studies, Miller said. Kelyn Soong The Washington Post, Arkansas Online, 27 Aug. 2023 This is important because circadian rhythms regulate nutrient processing, Dorothy Sears, PhD, professor of medicine at the University of California San Diego School of Medicine, told Health. Marnie Schwartz, Health, 25 Aug. 2023 Researchers recommend daytime light exposure and nighttime light avoidance to promote circadian rhythm development in infants.3 Move your baby's high chair or nursing pillow to the sunniest spot in the house and feed them there. Jana Banin, Parents, 24 Aug. 2023 But the studies on whether blue light affects circadian rhythms has mostly been on animals rather than humans, Kolla said. Kaitlin Sullivan, NBC News, 17 Aug. 2023 Torpor relies on circadian rhythms and might not have worked during the long, dark winter. Matt Hrodey, Discover Magazine, 16 Aug. 2023 See More

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'circadian.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History


Latin circa about + dies day + English -an entry 2 — more at deity

First Known Use

1959, in the meaning defined above

Time Traveler
The first known use of circadian was in 1959


Dictionary Entries Near circadian

Cite this Entry

“Circadian.” Dictionary, Merriam-Webster, Accessed 21 Sep. 2023.

Medical Definition


: being, having, characterized by, or occurring in approximately 24-hour periods or cycles (as of biological activity or function)
circadian periodicity
circadian rhythms in behavior or physiological activity
compare infradian, ultradian

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