Circadian Photoreception

Abstract
The brains of nonmammalian vertebrates contain populations of photoreceptive cells that are important for establishing the circadian rhythms of physiology and behavior. Do mammals, which evolved from strictly nocturnal ancestors, contain such photoreceptive cells? As [Menaker][1] explains in his Perspective, new work (including [Lucas][2] et al . and [Van Gelder][3] et al .) establishes that the mammalian retina contains photoreceptive ganglion cells carrying the photopigment melanopsin, which contribute to the entrainment of circadian rhythms to the light-dark cycle. [1]: http://www.sciencemag.org/cgi/content/full/299/5604/213 [2]: http://www.sciencemag.org/cgi/content/short/299/5604/245 [3]: http://www.sciencemag.org/cgi/content/short/299/5604/222