How Does Light Regulate Mood and Behavioral State?

The idea that light affects mood and behavioral state is not new. However, not much is known about the particular mechanisms and circuits involved. To fully understand these, we need to know what properties of light are important for mediating changes in mood as well as what photoreceptors and pathways are responsible. Increasing evidence from both human and animal studies imply that a specialized class of retinal ganglion cells, intrinsically photosensitive retinal ganglion cells (ipRGCs), plays an important role in the light-regulated effects on mood and behavioral state, which is in line with their well-established roles in other non-visual responses (pupillary light reflex and circadian photoentrainment). This paper reviews our current understanding on the mechanisms and paths by which the light information modulates behavioral state and mood.