[sur-key-dee-uhn, -kad-ee-, sur-kuh-dee-uhn]
noting or pertaining to rhythmic biological cycles recurring at approximately 24-hour intervals.

1955–60; < Latin circā about + di(ēs) day + -an Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
circadian (sɜːˈkeɪdɪən)
See also biological clock of or relating to biological processes that occur regularly at about 24-hour intervals, even in the absence of periodicity in the environment
[C20: from Latin circa about + diēs day]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

coined 1959 from L. circa "about" + diem, acc. sing. of dies "day" (see diurnal). The original use is in circadian rhythm.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

circadian cir·ca·di·an (sər-kā'dē-ən, -kād'ē-, sûr'kə-dī'ən, -dē'-)
Relating to biological variations or rhythms with a cycle of about 24 hours.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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Example sentences
Other brands contain melatonin, the hormone that maintains your body's
  circadian clock, but also can make you sleepy.
The profile of this pressure is believed to follow characteristic circadian
Sleep researchers have found that you can reset your circadian clock fairly
One of the brain centers that is thus permanently damaged is the circadian
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