Editing Circadian rhythm (section)

==Criteria==

To be called circadian, a biological rhythm must meet these three general criteria:<ref>{{cite book| vauthors = Johnson C |title=Chronobiology: Biological Timekeeping|year=2004|publisher=Sinauer Associates, Inc.|location=Sunderland, Massachusetts, USA|pages=67–105}}</ref>
# '''The rhythm has an endogenously derived free-running period of time that lasts approximately 24 hours.''' The rhythm persists in constant conditions, i.e. constant darkness, with a period of about 24 hours. The period of the rhythm in constant conditions is called the free-running period and is denoted by the Greek letter τ (tau). The rationale for this criterion is to distinguish circadian rhythms from simple responses to daily external cues. A rhythm cannot be said to be [[endogenous]] unless it has been tested and persists in conditions without external periodic input. In diurnal animals (active during daylight hours), in general τ is slightly greater than 24 hours, whereas, in nocturnal animals (active at night), in general τ is shorter than 24 hours.{{Citation needed|date=November 2024}}
# '''The rhythms are entrainable.''' The rhythm can be reset by exposure to external stimuli (such as light and heat), a process called [[Entrainment (chronobiology)|entrainment]]. The external stimulus used to entrain a rhythm is called the [[zeitgeber]], or "time giver".  Travel across [[time zone]]s illustrates the ability of the human biological clock to adjust to the local time; a person will usually experience [[jet lag]] before entrainment of their circadian clock has brought it into sync with local time.
# '''The rhythms exhibit temperature compensation.''' In other words, they maintain circadian periodicity over a range of physiological temperatures. Many organisms live at a broad range of temperatures, and differences in thermal energy will affect the [[Chemical kinetics|kinetics]] of all molecular processes in their {{Not a typo|cell(s)}}. In order to keep track of time, the organism's circadian clock must maintain roughly a 24-hour periodicity despite the changing kinetics, a property known as temperature compensation. The [[Q10 (temperature coefficient)|Q<sub>10</sub> temperature coefficient]] is a measure of this compensating effect. If the Q<sub>10</sub> coefficient remains approximately 1 as temperature increases, the rhythm is considered to be temperature-compensated.