Seasonal affective disorder is a form of depression that occurs during fall or winter, rarely during spring or summer. Seasonal affective disorder is more common in women than in men.   SAD occurs when the long summer days are cut short due to daylight saving time. The body has a decrease in serotonin, a neurotransmitter that stimulates the brain to feel happy when we're sad.   SAD may also result from increased melatonin, a hormone that triggers sleep. The brain secretes thi