Editing Circadian rhythm (section)

===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>