[dumbmatter]

https://en.wikipedia.org/wiki/Suprachiasmatic_nucleus

The SCN receives input from specialized photosensitive ganglion cells in the retina via the retinohypothalamic tract. Neurons in the ventrolateral SCN (vlSCN) have the ability for light-induced gene expression. Melanopsin-containing ganglion cells in the retina have a direct connection to the ventrolateral SCN via the retinohypothalamic tract. When the retina receives light, the vlSCN relays this information throughout the SCN allowing entrainment, synchronization, of the person's or animal's daily rhythms to the 24-hour cycle in nature.

[GraffitiTim]

And importantly, the photosensitive ganglion cells are most sensitive to blue wavelengths of light, which is why a lot of people recommend avoiding blue light at night.