Intrinsically photosensitive ganglion cells (melanopsin cells) in the eyes absorb short-wavelength light and communicate with the pineal gland, influencing melatonin release.

“And if you understand that light arriving on the eyes is absorbed by a particular cell type called the intrinsically photosensitive ganglion cell. It's just a name. You don't need to know the name, but if you want, it's the so-called intrinsically photosensitive ganglion cell, also called the melanopsin cell because it contains an opsin, a photopigment that absorbs shortwave length light that arrives through sunlight. Those cells communicate to particular stations in the brain that in turn connect to your so-called pineal gland, which is this little pea-sized gland in the middle of your brain that releases a hormone called melatonin.”