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

Poor lighting, particularly harsh artificial blue light, can impair leaders’ moods, disrupt their circadian rhythms, and elevate stress levels. —Julian Hayes Ii, Forbes.com, 30 June 2025

Improves Sleep Cherries are rich in melatonin, a natural compound that helps regulate the circadian rhythm (the body's natural sleep-wake cycle).8 Drinking tart cherry juice before bed can increase melatonin levels, improving sleep quality and duration.910 However, additional research is needed. —Cristina Mutchler, Verywell Health, 23 June 2025

Half of all meetings occur between 9:00 a.m. and 11:00 a.m. and 1:00 p.m. and 3:00 p.m., precisely when neuroscientists identify natural productivity peaks due to circadian rhythms. —Caroline Castrillon, Forbes.com, 18 June 2025

Wellness takes priority on every Four Seasons jet trip: All itineraries fly west to minimize jet lag (westward travel aligns better with the body’s circadian rhythm), and there are no overnight flights, to ensure guests enjoy a dreamy night’s sleep in their Four Seasons beds. —Alexandra Kirkman, Fortune, 18 June 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

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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