The specific cause of seasonal affective disorder remains unknown. It's likely, as with many mental health conditions, that genetics, age and perhaps most importantly, your body's natural chemical makeup all play a role in developing seasonal affective disorder.
Specifically, the culprits may include:
Your circadian rhythm: Some researchers suspect that the reduced level of sunlight in fall and winter may disrupt the circadian rhythm in certain people. The circadian rhythm is a physiological process that helps regulate your body's internal clock — letting you know when to sleep or wake. Disruption of this natural body clock may cause depression.
Melatonin: Some researchers theorize that seasonal affective disorder may be tied to melatonin, a sleep-related hormone that, in turn, has been linked to depression. The body's production of melatonin usually increases during the long nights of winter.
Serotonin: Other research suggests that a lack of serotonin, a natural brain chemical (neurotransmitter) that affects mood, may play a role. Reduced sunlight can cause a drop in serotonin, perhaps leading to depression.
Luckily, many treatments such as light therapy, medication and psychotherapy have been shown to aids those struggling with SAD.
(Taken from http://www.mayoclinic.com/health/seasonal-affective-disorder/DS00195)
