Seasonal affective disorder (SAD) is believed to be caused by a disturbance in the normal circadian rhythm of the body. Light entering through the eyes influences this rhythm. When it is dark, the pineal gland produces a substance called melatonin which is responsible for the drowsiness we feel each day after dusk. Light entering the eyes at dawn shuts off the production of melatonin. During the shorter days of winter, when people may rise before dawn or not leave their offices until after sunset, these normal rhythms may become disrupted, producing the symptoms of SAD.
There is also evidence linking SAD to a reduced amount of the neurotransmitter serotonin. Serotonin is a neurotransmitter involved in the regulation of mood that is increased by antidepressants called selective serotonin reuptake inhibitors (SSRIs). The disregulation or alteration in the serotonin system in the brain may be responsible for many of the symptoms of SAD, such as depression and carbohydrate cravings.
Miller, A.L. “Epidemiology, etiology, and natural treatment of seasonal affective disorder.” Alternative Medicine Review 10.1 (2006): 5-13.