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[sur-key-dee-uh n, -kad-ee-, sur-kuh-dee-uh n] /sɜrˈkeɪ di ən, -ˈkæd i-, ˌsɜr kəˈdi ən/
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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Examples from the web for circadian
  • 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 patterns.
  • Sleep researchers have found that you can reset your circadian clock fairly easily.
  • One of the brain centers that is thus permanently damaged is the circadian clock.
  • Once there, it can produce sleepiness either directly or by acting on the circadian clock.
  • circadian rhythms are almost impossible to move away from mentally.
  • The hormone melatonin regulates the circadian rhythm--the sleep cycle.
  • Reference to length of circadian rhythm also adjusted.
  • The same is true for circadian rhythms in fruit flies.
  • On a moon, eclipses would be part of a second circadian rhythm of sorts.
British Dictionary definitions for circadian


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
Word Origin
C20: from Latin circa about + diēs day
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for circadian

coined 1959 from Latin circa "about" (see circa) + diem, accusative singular of dies "day" (see diurnal). The original use is in circadian rhythm.

Online Etymology Dictionary, © 2010 Douglas Harper
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circadian in Medicine

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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