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

Did you know?

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

There is medical help for both circadian rhythm disorder and depression, and the mother should be urged to give it a try, if not for her own sake, then for her daughter's. —Arkansas Online, 17 Aug. 2025

Pathways involved in synaptic plasticity, circadian rhythm, and fatty acid metabolism were disrupted. —New Atlas, 16 Aug. 2025

Movement also helps regulate circadian rhythm, the body’s sleep-wake cycle, leading to better sleep. —Julia Ries Wexler, Health, 14 Aug. 2025

That includes lighting that will change to match with circadian rhythms throughout the day. —Joseph States, Chicago Tribune, 14 Aug. 2025

Word History

Etymology

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

First Known Use

1959, in the meaning defined above

Rhymes for circadian

• acadian
• akkadian
• arcadian
• palladian
• radian

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Medical Definition

circadian

adjective

cir·​ca·​di·​an (ˌ)sər-ˈkad-ē-ən, -ˈkād-; ˌsər-kə-ˈdī-ən, -ˈdē- 

: 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