Refers to events occurring within a 24-hour period, in the span of a full (24-hour) day, as in a circadian rhythm. The word “circadian” is a 20th-century invention. It was coined in 1959 from the Latin “circa” (around) + “diem” (a day). However, circadian rhythms have long been known. For example, the 18th-century Swedish botanist Carolus Linnaeus designed a garden that told time. He planted flowers in the garden that opened or closed their blossoms an hour apart around the clock. Circadian rhythmicity is a fundamental property possessed by all organisms. These rhythms are driven by an internal time-keeping system: a clock. Changes in the external environment, particularly in the light-dark cycle, entrain this biologic clock. Under constant environmental conditions devoid of time cues, rhythms driven by the clock show a period near, but usually not exactly equal to, 24 hours. Humans have an internal 24-hour clock which regulates our daily activities such as sleep and waking. Difficulties in readjusting our clock causes jet lag, work shift problems and some types of sleep disorders. Circadian clocks affect almost every level of our bodily functions. In 1997, a gene for the biological clock in a mouse was identified and cloned , the first such gene to be identified at the molecular level in a mammal. The identification of the “Clock” gene was proved by restoring a functioning biological clock to a line of mutant mice which had lost normal circadian rhythms. This was done by inserting DNA for the “Clock” gene into developing embryos that lacked it. They were born with a normal biological clock and passed the gene for it on to their descendants. The discovery of this gene regulating behavior was thus accompanied by simultaneous proof that the gene has been located by ‘rescuing’ the lost function of the gene. This research was reported in the journal Cell (vol. 89, pages 641-653 and 655-667, 1997).
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Relating to biologic variations or rhythms with a cycle of about 24 hours. Cf.:infradian, ultradian. [L. circa, about, + dies, day]

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cir·ca·di·an (.)sər-'kad-ē-ən, -'kād-; .sər-kə-'dī-ən, -'dē- adj being, having, characterized by, or occurring in approximately 24-hour periods or cycles (as of biological activity or function) <\circadian periodicity> <\circadian rhythms in behavior or physiological activity> compare INFRADIAN, ULTRADIAN

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denoting a biological rhythm or cycle of approximately 24 hours. Compare nyctohemeral; ultradian.

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cir·ca·di·an (sər-kaґde-ən) [L. circa about + dies a day] pertaining to a period of about 24 hours; applied especially to the rhythmic repetition of certain phenomena in living organisms at about the same time each day (circadian rhythm).

Medical dictionary. 2011.

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  • circadian — CIRCADIÁN adj. (med.; în sintagma) Ritm circadian = ritm biologic intern pe o durată de aproximativ 24 de ore al unui organism (aflat în imposibilitatea de a sesiza alternanţa dintre zi şi noapte). [pr.: di an] – Din fr. circadien. Trimis de hai …   Dicționar Român

  • circadian — (adj.) coined 1959 from L. circa about (see CIRCA (Cf. circa)) + diem, accusative singular of dies day (see DIURNAL (Cf. diurnal)). The original use is in circadian rhythm …   Etymology dictionary

  • circadian — *daily, diurnal, quotidian …   New Dictionary of Synonyms

  • circadian — ► ADJECTIVE ▪ (of biological processes) recurring on a twenty four hour cycle. ORIGIN from Latin circa about + dies day …   English terms dictionary

  • circadian — [sər kā′dē ən, sʉr΄kə dē′ən] adj. [coined < L circa, about + diem, acc. sing. of dies, day: see DEITY] Biol. designating or of behavioral or physiological rhythms associated with the 24 hour cycles of the earth s rotation, as, in man, the… …   English World dictionary

  • circadian — adjective Etymology: Latin circa about + dies day + English 2 an more at deity Date: 1959 being, having, characterized by, or occurring in approximately 24 hour periods or cycles (as of biological activity or function) < circadian rhythms in… …   New Collegiate Dictionary

  • circadian — adj. [L. circum, about; dies, day] Pertaining to a metabolic or behavorial phenomena in living organisms at about twenty four hour intervals; circadian rhythm; diurnal rhythm …   Dictionary of invertebrate zoology

  • circadian — adjective of, relating to, or showing rhythmic behaviour with a period of 24 hours; especially of a biological process The circadian clock is involved in the regulation of the diurnal sleep wake cycle, […] …   Wiktionary

  • circadian — adj. Circadian is used with these nouns: ↑clock, ↑rhythm …   Collocations dictionary

  • circadian — cir|ca|di|an [sə:ˈkeıdiən US sə:r ] adj [only before noun] technical [Date: 1900 2000; : Latin; Origin: circa around, about + dies day ] relating to a period of 24 hours, used especially when talking about changes in people s bodies ▪ the body s… …   Dictionary of contemporary English

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