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
Recent Examples on the Web
Planells added that eating within two hours of waking up helps stabilize blood sugar, kickstart metabolism, and align eating with your circadian rhythm.
—
Stephanie Brown, Verywell Health, 30 Sep. 2025
Possible explanations include changes in blood pressure, heart rate and even circadian rhythms (the body’s natural sleep-wake cycle).
—Lydia Patrick, MSNBC Newsweek, 30 Sep. 2025
When Coffee Has Its Biggest Impact The best time to indulge in that creamy latte or espresso shot often depends on your natural energy levels, your circadian rhythm, and how sensitive your body is to caffeine.
—Allison Forsyth, Health, 30 Sep. 2025
The fall equinox affects many things on Earth, including:
Human health: Our biological circadian rhythm is influenced by daylight variations, and this can result in disruptions to our sleep and mood patterns.
—Janet Loehrke, USA Today, 19 Sep. 2025
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Stephanie Brown, Verywell Health, 30 Sep. 2025
Possible explanations include changes in blood pressure, heart rate and even circadian rhythms (the body’s natural sleep-wake cycle).
—Lydia Patrick, MSNBC Newsweek, 30 Sep. 2025
When Coffee Has Its Biggest Impact The best time to indulge in that creamy latte or espresso shot often depends on your natural energy levels, your circadian rhythm, and how sensitive your body is to caffeine.
—Allison Forsyth, Health, 30 Sep. 2025
The fall equinox affects many things on Earth, including:
Human health: Our biological circadian rhythm is influenced by daylight variations, and this can result in disruptions to our sleep and mood patterns.
—Janet Loehrke, USA Today, 19 Sep. 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 3 Oct. 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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