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

Shorter days alter light exposure, which affects circadian rhythms, the internal biological processes that regulate sleep, alertness and mood. — Hanna Wickes, Sacbee.com, 23 Oct. 2026

Why seasons change affects your mood As daylight hours decrease in fall and winter, changes in light exposure disrupt circadian rhythms, the internal biological processes that regulate sleep, alertness and mood. — Hanna Wickes, Fort Worth Star-Telegram, 23 Oct. 2026

What’s Happening Shorter days are already disrupting circadian rhythms, the internal biological processes that regulate sleep, alertness and mood. — Hanna Wickes, Miami Herald, 23 Oct. 2026

Research on whether melatonin improves mood in young people with circadian depression is currently underway, says Tonini. — Tom Gavin, EverydayHealth.com, 21 Oct. 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