Editing Circadian rhythm (section)

==Importance in animals==
Circadian rhythmicity is present in the sleeping and feeding patterns of animals, including human beings. There are also clear patterns of core body temperature, [[Neural oscillation|brain wave]] activity, [[hormone]] production, cell regeneration, and other biological activities. In addition, [[photoperiodism]], the physiological reaction of organisms to the length of day or night, is vital to both plants and animals, and the circadian system plays a role in the measurement and interpretation of day length. Timely prediction of seasonal periods of weather conditions, food availability, or predator activity is crucial for survival of many species. Although not the only parameter, the changing length of the photoperiod (day length) is the most predictive environmental cue for the seasonal timing of physiology and behavior, most notably for timing of migration, hibernation, and reproduction.<ref>{{MEDRS|date=November 2013}}  {{cite web |title=Clock Tutorial #16: Photoperiodism – Models and Experimental Approaches (original work from 2005-08-13) |url=http://scienceblogs.com/clock/2007/07/clock_tutorial_16_photoperiodi_1.php |access-date=2007-12-09 | vauthors = Zivkovic BC |date=2007-07-25 |work=A Blog Around the Clock |publisher=ScienceBlogs |archive-url= https://web.archive.org/web/20080101142300/http://scienceblogs.com/clock/2007/07/clock_tutorial_16_photoperiodi_1.php |archive-date=2008-01-01 |url-status=dead }}</ref>

===Effect of circadian disruption===
[[Mutation]]s or [[Deletion (genetics)|deletions]] of [[CLOCK|clock]] genes in mice have demonstrated the importance of body clocks to ensure the proper timing of cellular/metabolic events; clock-mutant mice are [[Polyphagia|hyperphagic]] and obese, and have altered glucose metabolism.<ref name="pmid15845877">{{primary source inline|date=December 2013}} {{cite journal | vauthors = Turek FW, Joshu C, Kohsaka A, Lin E, Ivanova G, McDearmon E, Laposky A, Losee-Olson S, Easton A, Jensen DR, Eckel RH, Takahashi JS, Bass J | title = Obesity and metabolic syndrome in circadian Clock mutant mice | journal = Science | volume = 308 | issue = 5724 | pages = 1043–1045 | date = May 2005 | pmid = 15845877 | pmc = 3764501 | doi = 10.1126/science.1108750 | bibcode = 2005Sci...308.1043T }}</ref> In mice, deletion of the [[Rev-ErbA alpha]] clock gene can result in diet-induced [[obesity]] and changes the balance between [[glucose]] and lipid utilization, predisposing to [[diabetes]].<ref name="pmid22562834">{{cite journal | vauthors = Delezie J, Dumont S, Dardente H, Oudart H, Gréchez-Cassiau A, Klosen P, Teboul M, Delaunay F, Pévet P, Challet E | title = The nuclear receptor REV-ERBα is required for the daily balance of carbohydrate and lipid metabolism | journal = FASEB Journal | volume = 26 | issue = 8 | pages = 3321–3335 | date = August 2012 | pmid = 22562834 | doi = 10.1096/fj.12-208751 | doi-access = free | s2cid = 31204290 }}</ref> However, it is not clear whether there is a strong association between clock gene polymorphisms in humans and the susceptibility to develop the metabolic syndrome.<ref name="pmid22828941">{{primary source inline|date=December 2013}} {{cite journal | vauthors = Delezie J, Dumont S, Dardente H, Oudart H, Gréchez-Cassiau A, Klosen P, Teboul M, Delaunay F, Pévet P, Challet E | title = The nuclear receptor REV-ERBα is required for the daily balance of carbohydrate and lipid metabolism | journal = FASEB Journal | volume = 26 | issue = 8 | pages = 3321–3335 | date = August 2012 | pmid = 22562834 | doi = 10.1096/fj.12-208751 | doi-access = free | s2cid = 31204290 }}</ref><ref name="pmid18071340">{{primary source inline|date=December 2013}} {{cite journal | vauthors = Scott EM, Carter AM, Grant PJ | title = Association between polymorphisms in the Clock gene, obesity and the metabolic syndrome in man | journal = International Journal of Obesity | volume = 32 | issue = 4 | pages = 658–662 | date = April 2008 | pmid = 18071340 | doi = 10.1038/sj.ijo.0803778 | doi-access = free }}</ref>

===Effect of light–dark cycle===
The rhythm is linked to the light–dark cycle. Animals, including humans, kept in total darkness for extended periods eventually function with a [[free-running sleep|free-running]] rhythm. Their sleep cycle is pushed back or forward each "day", depending on whether their "day", their [[endogenous]] period, is shorter or longer than 24 hours. The environmental cues that reset the rhythms each day are called zeitgebers.<ref name=health.am>{{MEDRS|date=November 2013}}  {{cite web |vauthors=Shneerson JM, Ohayon MM, Carskadon MA |title=Circadian rhythms |publisher=Armenian Medical Network |work=Rapid eye movement (REM) sleep |url=http://www.sleep.health.am/sleep/more/circadian-rhythms/ |year=2007 |access-date=2007-09-19 |archive-date=2007-10-14 |archive-url=https://web.archive.org/web/20071014050810/http://www.sleep.health.am/sleep/more/circadian-rhythms/ |url-status=dead }}</ref> Totally blind subterranean mammals (e.g., [[blind mole rat]] ''Spalax'' sp.) are able to maintain their endogenous clocks in the apparent absence of external stimuli. Although they lack image-forming eyes, their [[Photoreceptor cell|photoreceptors]] (which detect light) are still functional; they do surface periodically as well.{{page needed|date=November 2013}}<ref>"The Rhythms of Life: The Biological Clocks That Control the Daily Lives of Every Living Thing" Russell Foster & Leon Kreitzman, Publisher: Profile Books Ltd.</ref>

Free-running organisms that normally have one or two consolidated sleep episodes will still have them when in an environment shielded from external cues, but the rhythm is not entrained to the 24-hour light–dark cycle in nature. The sleep–wake rhythm may, in these circumstances, become out of phase with other circadian or [[ultradian]] rhythms such as metabolic, hormonal, CNS electrical, or [[neurotransmitter]] rhythms.<ref>{{MEDRS|date=November 2013}} {{cite journal | vauthors = Regestein QR, Pavlova M | title = Treatment of delayed sleep phase syndrome | journal = General Hospital Psychiatry | volume = 17 | issue = 5 | pages = 335–345 | date = September 1995 | pmid = 8522148 | doi = 10.1016/0163-8343(95)00062-V }}</ref>

Recent research has influenced the design of [[human spaceflight|spacecraft]] environments, as systems that mimic the light–dark cycle have been found to be highly beneficial to astronauts.{{MEDRS|date=November 2013}}<ref>{{cite web | url=http://www.space.com/18917-astronauts-insomnia-light-bulbs.html | vauthors = Howell E | title=Space Station to Get New Insomnia-Fighting Light Bulbs | website = [[Space.com]] | date=14 December 2012 | access-date=2012-12-17}}</ref> [[Light therapy]] has been trialed as a [[Circadian rhythm sleep disorder#Treatment|treatment for sleep disorders]].

===Arctic animals===
Norwegian researchers at the [[University of Tromsø]] have shown that some [[Arctic#Flora and fauna|Arctic animals]] (e.g., [[Rock Ptarmigan|ptarmigan]], [[reindeer]]) show circadian rhythms only in the parts of the year that have daily sunrises and sunsets. In one study of reindeer, animals at [[70th parallel north|70 degrees North]] showed circadian rhythms in the autumn, winter and spring, but not in the summer.  Reindeer on [[Svalbard]] at [[78th parallel north|78 degrees North]] showed such rhythms only in autumn and spring. The researchers suspect that other Arctic animals as well may not show circadian rhythms in the constant light of summer and the constant dark of winter.<ref>{{primary source inline|date=November 2013}}  {{Cite news | vauthors = Spilde I |title=Reinsdyr uten døgnrytme |url=http://www.forskning.no/Artikler/2005/desember/1135264557.29 |publisher=forskning.no |date=December 2005 |access-date=2007-11-24 |language=nb |quote=...så det ikke ut til at reinen hadde noen døgnrytme om sommeren. Svalbardreinen hadde det heller ikke om vinteren. |archive-url=https://web.archive.org/web/20071203214441/http://www.forskning.no/Artikler/2005/desember/1135264557.29 |archive-date=2007-12-03 |url-status=dead }}</ref>

A 2006 study in northern Alaska found that day-living [[ground squirrel]]s and nocturnal [[porcupine]]s strictly maintain their circadian rhythms through 82 days and nights of sunshine. The researchers speculate that these two rodents notice that the apparent distance between the sun and the horizon is shortest once a day, and thus have a sufficient signal to entrain (adjust) by.<ref>{{cite journal |title=Mammalian activity – rest rhythms in Arctic continuous daylight |journal=Biological Rhythm Research |date=2006-12-01 | vauthors = Folk GE, Thrift DL, Zimmerman MB, Reimann P |s2cid=84625255 |volume=37 |issue=6 |pages=455–469 |doi=10.1080/09291010600738551 |bibcode=2006BioRR..37..455F |quote=Would local animals maintained under natural continuous daylight demonstrate the Aschoff effect described in previously published laboratory experiments using continuous light, in which rats' circadian activity patterns changed systematically to a longer period, expressing a 26-hour day of activity and rest? }}</ref>

===Butterflies and moths===
The navigation of the fall migration of the [[monarch butterfly|Eastern North American monarch butterfly]] (''Danaus plexippus'') to their overwintering grounds in central Mexico uses a time-compensated sun compass that depends upon a circadian clock in their antennae.<ref>{{primary source inline|date=November 2013}} {{cite journal | vauthors = Merlin C, Gegear RJ, Reppert SM | title = Antennal circadian clocks coordinate sun compass orientation in migratory monarch butterflies | journal = Science | volume = 325 | issue = 5948 | pages = 1700–1704 | date = September 2009 | pmid = 19779201 | pmc = 2754321 | doi = 10.1126/science.1176221 | bibcode = 2009Sci...325.1700M }}</ref><ref>{{primary source inline|date=November 2013}} {{cite journal | vauthors = Kyriacou CP | title = Physiology. Unraveling traveling | journal = Science | volume = 325 | issue = 5948 | pages = 1629–1630 | date = September 2009 | pmid = 19779177 | doi = 10.1126/science.1178935 | s2cid = 206522416 }}</ref> Circadian rhythm is also known to control mating behavioral in certain moth species such as ''[[Spodoptera littoralis]]'', where females produce specific [[pheromone]] that attracts and resets the male circadian rhythm to induce mating at night.<ref>{{cite journal | vauthors = Silvegren G, Löfstedt C, Qi Rosén W | title = Circadian mating activity and effect of pheromone pre-exposure on pheromone response rhythms in the moth Spodoptera littoralis | journal = Journal of Insect Physiology | volume = 51 | issue = 3 | pages = 277–286 | date = March 2005 | pmid = 15749110 | doi = 10.1016/j.jinsphys.2004.11.013 | bibcode = 2005JInsP..51..277S }}</ref>