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circadian \sur-KEY-dee-uhn, -KAD-ee-, sur-kuh-DEE-uhn\,
adjective:
noting or pertaining to rhythmic biological cycles recurring at approximately 24-hour intervals.

My circadian clock, which puts me to sleep at night and wakes me up in the morning in a regular twenty-four-hour fashion, has a larger arc that seems set at twenty-four weeks.
-- Jeanette Winterson, "Written on the Body," 1993

Even a guard on night duty succumbs to the body's natural circadian rhythms and would be far from alert.
-- Clive Cussler, "The Jungle," 2010

Circadian was first used to apply to the "circadian rhythms" of the body, primarily our daily internal cycles of hunger, rest and wakefulness. It comes from the Latin roots circā meaning "about" and di meaning day.

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