Light exposure activates melanopsin cells, which inhibit melatonin production.

“Light arriving on the eyes is absorbed by a particular cell type called the intrinsically photosensitive ganglen 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 ganglen cell, also called the melanopsin cell, because it contains an opsin, a photo pigment that absorbs shortwavelength light that arrives through sunlight. Those cells communicate to particular stations in the brain that in turn can connect to your so-called pineal gland, which is this little P-sized gland in the middle of your brain that releases a hormone called melatonin.”