Circadian photoreceptor in mammals is a kind of specific retinal ganglion cells which are light sensitive. Melanopsin has been proposed as an important photoreceptive molecule for the mammalian circadian system. Circadian photoreceptors are functionally characterized by the direct sensitivity to light with broad spectrum and the relatively high stability. The major circadian pacemaker in the hypothalamic suprachiasmatic nucleus is entrained to the light/dark cycles from the outside world by circadian photoreceptors.