Making sense of our world and responding appropriately requires sophisticated brain circuits. These circuits form by utilizing cues from the environment. A potent cue is light, yet investigators have believed that its influence occurs late in development because the rods and cones—long held to be the only photoreceptors in mammals—only become functional in the second week after birth. Now researchers have discovered additional mammalian photoreceptors, called intrinsically photosensitive retinal ganglion cells (ipRGCs). They capture light with a receptor called melanopsin and signal environmental light levels to the brain, at birth and possibly even before. Adult mice that have lacked melanopsin throughout life show abnormal responses to light, which disrupt their internal body clock and other essential functions. We are determining the role of ipRGC signaling in the development of the nervous system. Our hypothesis is that light exerts an early and potent influence on development through ipRGCs and that this influence is required for the normal formation of physiology and behavior. We are testing this hypothesis by determining how ipRGCs respond to light in the developing mouse (Aim 1), how signals from ipRGCs influence neurons in the brain (Aim 2), and how the elimination of ipRGC signaling for intervals during development impacts physiology and behavior in adulthood (Aim 3).