from The American Heritage® Dictionary of the English Language, 4th Edition

  • adj. Biology Relating to or exhibiting approximately 24-hour periodicity.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • adj. of, relating to, or showing rhythmic behaviour with a period of 24 hours; especially of a biological process

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • adj. of or relating to biological processes occurring at 24-hour intervals


from The American Heritage® Dictionary of the English Language, 4th Edition

Latin circā, around; see circa + Latin diēs, day.

from Wiktionary, Creative Commons Attribution/Share-Alike License

From Latin circā ("about") and diēs ("a day") with the English suffix -an.


  • The term circadian rhythms refers to biological cycles that repeat about every twenty-four hours.

    Solve Your Child’s Sleep Problems

  • So I think clearly what we call circadian rhythms, these oscillating clocks within the body, within cells in the body, clearly exist.

    CNN Transcript Jun 24, 2006

  • "Circadian rhythms are the outward manifestation of an internal timing system which is driven by several genetic elements, what we call circadian clock genes."

    Medindia Health News

  • There is also the change in circadian rythmns, as I go from being a gainfully employed American to a retired one.

    Sleep Disorder Specialist MD?

  • The word circadian is derived from the Latin circa diem or about a day and refers to the mysterious twenty-four-hour rhythms which control all brain outputs-behavioral, autonomic or neuro-endocrine-of all animals or humans who live on Earth with its twenty-four-hour alternation of light and darkness.


  • Sleep patterns in children and adults follow what is known as the circadian rhythm, which determines how easily we fall asleep, stay asleep, wake up in the morning and maintain a level of alertness during the day. rss feed

  • Your body's clock, or body clock, is the time-keeping system inside you that controls your natural 24-hour sleep and wake cycle also known as your circadian rhythms.

    Hyungsoo Kim: Is It Possible To Re-Set Your Biological Clock?

  • It is called a circadian rhythm and if it is out of sync you get that feeling of being "so tired but I can't sleep" or you fall asleep at 6 p.m. on the couch, wake at 2 a.m. and wonder what happened.

    Dr. Michael J. Breus: Sleep Rituals: Training The Body And The Mind

  • Negative consequencesTeenager's biological rhythms - often referred to as their circadian rhythms - favor later bedtimes and later wake-up times.

    American Teenagers Dangerously Sleep Deprived

  • The genes regulating this so-called circadian clock leave traces of their activity throughout the body.

    Study Examines Sugar


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  • Interesting, the standard pronunciation really loses the etymology of circa-dian, around a day.

    April 13, 2009