Editing Circadian clock (section)

== Variation in circadian clocks ==

While a precise 24-hour circadian clock is found in many organisms, it is not universal. Organisms living in the high arctic or high antarctic do not experience solar time in all seasons, though most are believed to maintain a circadian rhythm close to 24 hours, such as bears during torpor.<ref name="pmid27660641">{{cite journal | vauthors = Jansen HT, Leise T, Stenhouse G, Pigeon K, Kasworm W, Teisberg J, Radandt T, Dallmann R, Brown S, Robbins CT | display-authors = 6 | title = The bear circadian clock doesn't 'sleep' during winter dormancy | journal = Frontiers in Zoology | volume = 13 | pages = 42 | date = 2016 | pmid = 27660641 | pmc = 5026772 | doi = 10.1186/s12983-016-0173-x | doi-access = free }}</ref> Much of the earth's biomass resides in the dark biosphere, and while these organisms may exhibit rhythmic physiology, for these organisms the dominant rhythm is unlikely to be circadian.<ref name="pmid27263116">{{cite journal | vauthors = Beale AD, Whitmore D, Moran D | title = Life in a dark biosphere: a review of circadian physiology in "arrhythmic" environments | journal = Journal of Comparative Physiology B: Biochemical, Systemic, and Environmental Physiology | volume = 186 | issue = 8 | pages = 947–968 | date = December 2016 | pmid = 27263116 | pmc = 5090016 | doi = 10.1007/s00360-016-1000-6 }}</ref> For east-west migratory organisms—and especially those organisms that circumnavigate the globe<!-- circumnavigating whales at 40°S ??? -->—the absolute 24-hour phase might deviate over months, seasons, or years.

Some spiders exhibit unusually long or short circadian clocks. Some [[Cyclosa|trashline orbweavers]], for example, have 18.5-hour circadian clocks, but are still able to entrain to a 24-hour cycle. This adaptation may help the spiders avoid predators by allowing them to be most active before sunrise.<ref name="Smith2017">{{cite web| vauthors = Smith DG |title=Meet the Spiders That Completely Defy What We Know as Jet Lag|url=https://www.scientificamerican.com/article/meet-the-spiders-that-completely-defy-what-we-know-as-jet-lag/|website=Scientific American|access-date=21 November 2017|date=21 November 2017}}</ref> [[Latrodectus|Black widows]]' clocks are arrhythmic, perhaps due to their preference for dark environments.<ref name="Guarino">{{cite news| vauthors = Guarino B |title='It's crazy': These animals have the fastest circadian clocks found in nature|url=https://www.washingtonpost.com/news/speaking-of-science/wp/2017/11/14/its-crazy-these-animals-have-the-fastest-circadian-clocks-found-in-nature/|access-date=21 November 2017|newspaper=Washington Post|date=14 November 2017}}</ref>