circadian
adjective
cir·ca·di·an
sər-ˈkā-dē-ən 
: 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
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But there may be other factors in play, beyond disrupting the circadian clock.
—
Veronique Greenwood, Time, 9 Dec. 2025
Other significant features on the jet, which as the longest seat length in its class, include a Pur Air system that taps up the cabin air every 90 seconds and a Soleil circadian-rhythm system that adjusts light to reduce fatigue.
—Nicole Hoey, Robb Report, 9 Dec. 2025
For higher sleep quality, sleep pressure and your circadian rhythm should be working together.
—Gina Park, CNN Money, 8 Dec. 2025
The concept is supported by research into circadian rhythms and the body’s natural blood-pressure patterns.
—Kelly McGreal, FOXNews.com, 4 Dec. 2025
See All Example Sentences for circadian
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Veronique Greenwood, Time, 9 Dec. 2025
Other significant features on the jet, which as the longest seat length in its class, include a Pur Air system that taps up the cabin air every 90 seconds and a Soleil circadian-rhythm system that adjusts light to reduce fatigue.
—Nicole Hoey, Robb Report, 9 Dec. 2025
For higher sleep quality, sleep pressure and your circadian rhythm should be working together.
—Gina Park, CNN Money, 8 Dec. 2025
The concept is supported by research into circadian rhythms and the body’s natural blood-pressure patterns.
—Kelly McGreal, FOXNews.com, 4 Dec. 2025
See All Example Sentences for circadian
Cite this Entry
“Circadian.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/circadian. Accessed 17 Dec. 2025.
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circadian
adjective
cir·ca·di·an
(ˌ)sər-ˈkad-ē-ən, -ˈkād-; ˌsər-kə-ˈdī-ən, -ˈdē-
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