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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The company’s products include peel-and-stick photo tiles made from digital images, self-watering plants designed to be refilled monthly, and smart lighting that adjusts to natural circadian rhythms.
—
Connie Etemadi, USA Today, 9 Feb. 2026
Many people who are more active at night, stay up and wake later are subject to circadian misalignment, meaning their internal clocks aren’t necessarily compatible with real-world demands like waking up early for work.
—Helen Carefoot, Flow Space, 9 Feb. 2026
Some mouse studies have suggested that immune cells seem to follow a circadian rhythm, with some cells trafficking through the blood more at different times.
—Angus Chen, STAT, 2 Feb. 2026
According to the study, circadian misalignment can potentially promote unhealthy lifestyle behaviors such as irregular sleep patterns, poor diet, heavy alcohol drinking, and smoking.
—Michele Laufik, Martha Stewart, 30 Jan. 2026
See All Example Sentences for circadian
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Connie Etemadi, USA Today, 9 Feb. 2026
Many people who are more active at night, stay up and wake later are subject to circadian misalignment, meaning their internal clocks aren’t necessarily compatible with real-world demands like waking up early for work.
—Helen Carefoot, Flow Space, 9 Feb. 2026
Some mouse studies have suggested that immune cells seem to follow a circadian rhythm, with some cells trafficking through the blood more at different times.
—Angus Chen, STAT, 2 Feb. 2026
According to the study, circadian misalignment can potentially promote unhealthy lifestyle behaviors such as irregular sleep patterns, poor diet, heavy alcohol drinking, and smoking.
—Michele Laufik, Martha Stewart, 30 Jan. 2026
See All Example Sentences for circadian
Cite this Entry
“Circadian.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/circadian. Accessed 13 Feb. 2026.
Copy CitationMedical Definition
circadian
adjective
cir·ca·di·an
(ˌ)sər-ˈkad-ē-ən, -ˈkād-; ˌsər-kə-ˈdī-ən, -ˈdē-
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