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

Research suggests the body’s circadian rhythm—the internal clock that regulates sleep, hormones, and metabolism—is partly influenced by meal timing. — Trista Chan, HEALTH, 17 Aug. 2026

The system syncs with natural circadian rhythms and connects you with the outdoors. — Better Homes & Gardens, 14 Aug. 2026

Previous studies have examined individual relationships, such as the effects of lighting on sleep and circadian rhythms or the influence of layout on privacy and social interaction. — Atharva Gosavi, Interesting Engineering, 12 Aug. 2026

Chinese medicine taught me that our symptoms are signals, not inconveniences to silence, and that food as thy medicine, sleep, circadian rhythms, stress awareness and our daily habits can all be powerful forms of medicine. — Tanya Akim, Forbes.com, 12 Aug. 2026

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

• radian
• acadian
• akkadian
• arcadian
• palladian

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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