A melanopsin photopigment system in the retina is sensitive to light intensity and signals the brain about ambient brightness, playing a role in the circadian system.

“Uh, and the thing that's really peculiar about this pigment is that it's in the wrong place. In a sense, when you think about the structure of the retina, you think about a layer cake essentially. You've got this this thin membrane at the back of your eye, but it's actually a stack of thin layers. And the outermost of those layers is where these photo receptors you were talking about earlier are sitting. That's where the film of your camera is essentially. That's where the photons do their magic with the photo pigments and turn it into a neural signal.”