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

Melatonin Melatonin is a hormone that occurs naturally, which is responsible for the timing of the circadian rhythm, or your sleep cycle. —Megan Nunn, Verywell Health, 28 Jan. 2026

Though the time change allows for more daylight in the spring and summer evenings, many have argued the change disturbs the body's circadian rhythms and has significant health drawbacks. —Maia Pandey, jsonline.com, 28 Jan. 2026

Structured work intervals aligned with natural circadian rhythms increases cognitive performance by up to 40% compared to unstructured work patterns. —Bryan Robinson, Forbes.com, 22 Jan. 2026

There’s melatonin, for example — which regulates circadian rhythms and sleep. —Gretta Monahan, Boston Herald, 18 Jan. 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

• 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