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

Frequent travel across multiple time zones disrupts circadian rhythms and increases the risk of injuries due to inadequate rest and recovery. —Claire Poole, Forbes.com, 5 Apr. 2025

The time change also brings with it other adverse health impacts due to changes in our circadian rhythm. —Simmone Shah, TIME, 5 Mar. 2025

They are evolutionarily wired for risk-taking during teenage years as their circadian rhythms shift and reward systems develop. —Rachel Hale, USA Today, 26 Mar. 2025

Your circadian rhythm — the cycle that regulates sleep, hormone release and temperature — might be out of whack. —Andrew Torgan, CNN, 23 Mar. 2025

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

• acadian
• akkadian
• arcadian
• palladian
• radian

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