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

My suite turned out to be on the property’s wellness floor, where things like hypoallergenic hardwood flooring, water-purification systems, and lighting that adapts to your circadian rhythm are all standard, as are Alo yoga mats and blocks. — Chadner Navarro, Travel + Leisure, 8 Sep. 2026

But does eating in sync with your circadian rhythm actually do anything for your health? — Maggie Donahue, HEALTH, 1 Sep. 2026

Confining food intake to a consistent daytime window also synchronizes your peripheral metabolic clocks (body clocks in almost every organ) with your central circadian clock, which maintains your sleep cycle. — Teresa Maalouf, Verywell Health, 30 Aug. 2026

Some of the bodily clocks that affect you every day are the sleep-wake, or circadian, rhythm and your chronotype, which is the body’s natural timing preference—often described as being either an early bird or a night owl. — Masha Remskar, Scientific American, 29 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

Podcast

Get Word of the Day delivered to your inbox!

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