circadian

American  

[sur-key-dee-uhn] / sɜrˈkeɪ di ən /

adjective

Physiology.

1. of or relating to a biorhythm or regular biological cycle occurring once every 24 hours, such as waking and sleeping.

   Exposure to blue light at night can disrupt your circadian rhythm, making your biological clock think it's earlier in the day rather than time for bed.

   He is doing research related to the circadian cycling of hormones.

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circadian British  

/ sɜːˈkeɪdɪən /

adjective

1. of or relating to biological processes that occur regularly at about 24-hour intervals, even in the absence of periodicity in the environment See also biological clock

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Etymology

Origin of circadian

First recorded in 1955–60; from circā “about” + Latin di(ēs) “day” + -an ( def. ); circa ( def. )

Explanation

Something that's circadian happens on a regular, daily basis. Your body's circadian rhythm is made up of processes that occur regularly every 24 hours. You'll find the word circadian in biology class, or your anatomy and physiology textbook, because it describes a biological process in the human body, as well as in other organisms. Your cycle of sleep, for example, happens on a circadian schedule — approximately every 24 hours, you become tired enough to lie down and close your eyes, a built-in rhythm that's affected somewhat by light. Circadian comes from Latin roots, circa, "about," and diem, "day."

Example Sentences

The study, carried out with Mia Morrell, an NYU graduate student, and Leela Elliott, an NYU undergraduate, may also help scientists better understand biological timing systems such as circadian rhythms.

From Science Daily • Mar. 22, 2026

However, sleep scientists and medical groups generally argue in favor of permanent standard time, saying it better aligns with human circadian rhythms and reduces health risks.

From Los Angeles Times • Mar. 7, 2026

“I’ve noticed it makes me very low energy because my circadian rhythm relies on the sunrise to really wake up,” said Hannah M. Le, founder of Buckle Scrunchies, a hair-tie brand.

From MarketWatch • Mar. 6, 2026

Robertson says light pollution is increasing, which affects human circadian rhythms, and the lives of other species.

From BBC • Feb. 28, 2026

Not only was this arrangement bad for the astronaut’s circadian rhythm, but the astronaut who was on duty made enough noise as he went about his work to keep his partner awake.

From "Flying to the Moon: An Astronaut's Story" by Michael Collins

Related Words

• constantly
• day-to-day
• everyday
• often
• periodic
• regular
• regularly
• routine