Day length is converted into hormonal and chemical signals, including melatonin, dopamine, and stress hormones, which influence brain states.

“In long days where we get a lot of sunlight, both in our eyes and on our skin, melatonin levels are reduced. Melatonin is a hormone that tends to produce states of sleepiness and quiescence. It also tends to activate pathways that tend to reduce things like breeding and sexual behavior. In long days, dopamine is increased. Dopamine is a molecule associated with feelings of wellbeing and motivation and the desire to seek out all sorts of things, all sorts of motivated behaviors. And in long days provided we're getting enough sunlight on our skin and to our eyes, the stress hormones, especially cortisol, and some of the other stress hormones are reduced in levels.”