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

Blue-light blocking glasses McCarty says that blue-light blocking glasses are based on a legitimate biological concept (that blue light affects circadian rhythms and melatonin production). —Sharon Brandwein, USA Today, 2 June 2026

Fragmented sleep, irregular sleep timing, and disrupted circadian rhythms can all negatively affect your health. —Brian Mastroianni, Health, 2 June 2026

Your circadian system needs real intensity to set the body clock, and no household fixture comes close. —Allison Palmer june 2, Fort Worth Star-Telegram, 2 June 2026

The research on morning exercise and its connection to your body clock tells a similar story about how the same circadian system responds to consistent daily cues. —Allison Palmer june 2, Sacbee.com, 2 June 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