Zeitgeber

Zeitgeber is the German word for timer. It is used to denote any factor that may set the internal clock. The most important zeitgeber is light, which acts on the SCN through the eye via the retinohypothalamic tract. In the SCN is the central internal clock that controls a large number of circadian rhythms. Other zeitgebers are in the liver for example and these too control circadian rhythms.