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

This Works formulations are grounded in Functional Fragrances, circadian biology and neurosensory rituals, engineered to down-regulate arousal and support the natural circadian cycle. —ABC News, 18 Apr. 2026

Rapid changes in daylight can affect your circadian rhythm and hormone levels, which can leave you feeling off. —Eleesha Lockett, Verywell Health, 14 Apr. 2026

Light triggers vitamin D production and is a crucial driver of circadian rhythms. —Maria Godoy, NPR, 13 Apr. 2026

Dim red lights are less likely to shift circadian rhythms than white or blue light and make a better nighttime alternative for your bedroom. —Allison Palmer, Miami Herald, 13 Apr. 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