Circadian Photoreception

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 explains in his Perspective, new work (including Lucas et al. and Van Gelder 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.