Overhead lights stimulate melanopsin ganglion cells, signaling the hypothalamus to promote alertness.

“What's special about overhead lights for setting alertness is that the neurons in our eyes, which are called melanopsin ganglion cells, that's the fancy name. Melanopsin ganglion cells are mainly enriched in the lower half of our retinas in our eyes and view the upper visual field. Those neurons send little wires to an area of our hypothalamus, right above the roof of our mouth, that creates a state of alertness.”