Circadian rhythm

<p>A&nbsp;<b >circadian rhythm</b>&nbsp;is any biological process that displays an<a href="http://en.wikipedia.org/wiki/Endogeny" title="Endogeny" >endogenous</a>,&nbsp;<a href="http://en.wikipedia.org/wiki/Entrainment_(chronobiology)" title="Entrainment (chronobiology)" >entrainable</a>&nbsp;oscillation of about 24 hours. These rhythms are driven by a&nbsp;<a href="http://en.wikipedia.org/wiki/Circadian_clock" title="Circadian clock" >circadian clock</a>, and rhythms have been widely observed in<a href="http://en.wikipedia.org/wiki/Plants" title="Plants" class="mw-redirect" >plants</a>,&nbsp;<a href="http://en.wikipedia.org/wiki/Animals" title="Animals" class="mw-redirect" >animals</a>,&nbsp;<a href="http://en.wikipedia.org/wiki/Fungi" title="Fungi" class="mw-redirect" >fungi</a>&nbsp;and&nbsp;<a href="http://en.wikipedia.org/wiki/Cyanobacteria" title="Cyanobacteria" >cyanobacteria</a>.&nbsp;</p>

<p>

The term&nbsp;<i >circadian</i>&nbsp;comes from the&nbsp;<a href="http://en.wikipedia.org/wiki/Latin" title="Latin" >Latin</a>&nbsp;<i ><a href="http://en.wikipedia.org/wiki/Circa" title="Circa" >circa</a></i>, meaning "around" (or "approximately"), and&nbsp;<i >diem</i>&nbsp;or&nbsp;<i >dies</i>, meaning "day". The formal study of biological temporal rhythms, such as daily,&nbsp;<a href="http://en.wikipedia.org/wiki/Tidal" title="Tidal" class="mw-redirect" >tidal</a>, weekly, seasonal, and annual rhythms, is called&nbsp;<a href="http://en.wikipedia.org/wiki/Chronobiology" title="Chronobiology" >chronobiology</a>.&nbsp;</p>

<p>

Although circadian rhythms are endogenous ("built-in", self-sustained), they are adjusted (entrained) to the local environment by external cues called <a href="http://en.wikipedia.org/wiki/Zeitgeber" title="Zeitgeber" >zeitgebers</a>, commonly the most important of which is&nbsp;<a href="http://en.wikipedia.org/wiki/Daylight" title="Daylight" >daylight</a>.</p>