“Our species is diurnal, and the best chance of finding sustenance and other rewards was in the light phase (think about the challenge of identifying edible berries or stalking a mammoth). Consequently, we are configured to be more alert during the day than at night. Consistent with the link between light and mood, some clinically serious low mood is triggered by the seasonal change of shorter daylight hours. The onset of seasonal affective disorder, a subtype of mood disorder, is usually in winter.”