Editing Circadian clock (section)

{{short description|Biological mechanism that controls circadian rhythm}}
A '''circadian clock''', or '''circadian oscillator''', also known as one’s '''internal alarm clock''' is a biochemical [[oscillator]] that cycles with a stable [[phase (waves)|phase]] and is synchronized with [[solar time]].

Such a clock's ''in vivo'' period is necessarily almost exactly 24 hours (the earth's current [[day|solar day]]). In most living organisms, internally synchronized circadian clocks make it possible for the organism to anticipate daily environmental changes corresponding with the day–night cycle and adjust its biology and behavior accordingly.

The term '''circadian''' derives from the Latin ''circa'' (about) ''dies'' (a day), since when taken away from external cues (such as environmental light), they do not run to exactly 24 hours. Clocks in humans in a lab in constant low light, for example, will average about 24.2 hours per day, rather than 24 hours exactly.<ref>{{cite journal |url= http://news.harvard.edu/gazette/1999/07.15/bioclock24.html | vauthors = Cromie W |title=Human Biological Clock Set Back an Hour |journal=Harvard Gazette |date=1999-07-15 |access-date=2015-07-29 }}</ref>

The normal body clock oscillates with an endogenous period of exactly 24 hours, it [[Entrainment (chronobiology)|entrains]], when it receives sufficient daily corrective signals from the environment, primarily daylight and darkness. Circadian clocks are the central mechanisms that drive [[circadian rhythm]]s. They consist of three major components:
* a central biochemical oscillator with a period of about 24 hours that keeps time;
* a series of input pathways to this central oscillator to allow [[entrainment (chronobiology)|entrainment]] of the clock;
* a series of output pathways tied to distinct phases of the oscillator that regulate overt rhythms in biochemistry, physiology, and behavior throughout an organism.

The clock is reset as an organism senses environmental time cues of which the primary one is light. Circadian oscillators are ubiquitous in tissues of the body where they are synchronized by both [[Endogeny (biology)|endogenous]] and external signals to regulate transcriptional activity throughout the day in a tissue-specific manner.<ref name="Ueda">{{cite journal | vauthors = Ueda HR, Hayashi S, Chen W, Sano M, Machida M, Shigeyoshi Y, Iino M, Hashimoto S | display-authors = 6 | title = System-level identification of transcriptional circuits underlying mammalian circadian clocks | journal = Nature Genetics | volume = 37 | issue = 2 | pages = 187–192 | date = February 2005 | pmid = 15665827 | doi = 10.1038/ng1504 | s2cid = 18112337 }}</ref> The circadian clock is intertwined with most cellular metabolic processes and it is affected by organism aging.<ref>{{cite journal | vauthors = Tevy MF, Giebultowicz J, Pincus Z, Mazzoccoli G, Vinciguerra M | title = Aging signaling pathways and circadian clock-dependent metabolic derangements | journal = Trends in Endocrinology and Metabolism | volume = 24 | issue = 5 | pages = 229–237 | date = May 2013 | pmid = 23299029 | pmc = 3624052 | doi = 10.1016/j.tem.2012.12.002 }}</ref> The basic molecular mechanisms of the biological clock have been defined in [[vertebrate]] species, ''[[Drosophila melanogaster]]'', [[TOC1 (gene)|plants]], [[fungi]], [[bacterial circadian rhythms|bacteria]],<ref name="pmid11687489">{{cite journal | vauthors = Harmer SL, Panda S, Kay SA | title = Molecular bases of circadian rhythms | journal = Annual Review of Cell and Developmental Biology | volume = 17 | pages = 215–253 | year = 2001 | pmid = 11687489 | doi = 10.1146/annurev.cellbio.17.1.215 }}</ref><ref name="Lowrey">{{cite journal | vauthors = Lowrey PL, Takahashi JS | title = Mammalian circadian biology: elucidating genome-wide levels of temporal organization | journal = Annual Review of Genomics and Human Genetics | volume = 5 | pages = 407–441 | year = 2004 | pmid = 15485355 | pmc = 3770722 | doi = 10.1146/annurev.genom.5.061903.175925 }}</ref> and presumably also in [[Archaea]].<ref name="Edgar_2012">{{cite journal | vauthors = Edgar RS, Green EW, Zhao Y, van Ooijen G, Olmedo M, Qin X, Xu Y, Pan M, Valekunja UK, Feeney KA, Maywood ES, Hastings MH, Baliga NS, Merrow M, Millar AJ, Johnson CH, Kyriacou CP, O'Neill JS, Reddy AB | display-authors = 6 | title = Peroxiredoxins are conserved markers of circadian rhythms | journal = Nature | volume = 485 | issue = 7399 | pages = 459–464 | date = May 2012 | pmid = 22622569 | pmc = 3398137 | doi = 10.1038/nature11088 | bibcode = 2012Natur.485..459E }}</ref><ref name="pmid12604787">{{cite journal | vauthors = Dvornyk V, Vinogradova O, Nevo E | title = Origin and evolution of circadian clock genes in prokaryotes | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 5 | pages = 2495–2500 | date = March 2003 | pmid = 12604787 | pmc = 151369 | doi = 10.1073/pnas.0130099100 | doi-access = free | bibcode = 2003PNAS..100.2495D }}</ref><ref name="pmid19424498">{{cite journal | vauthors = Whitehead K, Pan M, Masumura K, Bonneau R, Baliga NS | title = Diurnally entrained anticipatory behavior in archaea | journal = PLOS ONE | volume = 4 | issue = 5 | pages = e5485 | year = 2009 | pmid = 19424498 | pmc = 2675056 | doi = 10.1371/journal.pone.0005485 | doi-access = free | bibcode = 2009PLoSO...4.5485W }}</ref>

In 2017, the [[Nobel Prize in Physiology or Medicine]] was awarded to [[Jeffrey C. Hall]], [[Michael Rosbash]] and [[Michael W. Young]] "for their discoveries of molecular mechanisms controlling the circadian rhythm" in fruit flies.<ref name=nobel17>{{Cite web|url=https://www.nobelprize.org/nobel_prizes/medicine/laureates/2017/|title=The Nobel Prize in Physiology or Medicine 2017|website=www.nobelprize.org|access-date=2017-10-06}}</ref>