are endogenous ("built-in", self-sustained), they are adjusted ( entrained ) to the environment by external cues called zeitgebers , the primary one of which is daylight . source: http://en.wikipedia.org/wiki/Circadian_rhythm CLICK HERE TO CHECK OUT : Circadian Rhythms Fact Sheet source: http://universe-review.ca/F10-multicell.htm source: http://www.dot.state.ct.us/ShowPage.aspx?s=5&a=31 No comments: Email This BlogThis! Share to Twitter