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

Here’s everything to know about circadian lighting. —Allison Palmer, Kansas City Star, 12 May 2026

Key Takeaways Melatonin is most useful for circadian rhythm disorders and works best when used short-term and taken about an hour before bed. —Jessica Swirble, Verywell Health, 11 May 2026

The group recommends at least 250 lux of circadian-effective light during waking hours, a sharp drop in the evening and near-darkness overnight. —Allison Palmer, Charlotte Observer, 9 May 2026

The 2017 study landed on just 10 minutes of low-to-moderate stair walking as enough to produce a measurable energy boost, and outdoor walking adds the light exposure that anchors your circadian rhythm. —Samantha Agate, Miami Herald, 6 May 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