Seasonal affective disorder. Artwork of a man affected by seasonal affective disorder (SAD) joyously holding a light bulb aloft. SAD is a form of depression brought on by decreased hours of daylight. It therefore begins to manifest in late autumn, and naturally subsides in the spring. During the winter months, sufferers feel withdrawn and tired. It has been shown that phototherapy (exposure to bright lights) has a definite positive effect in reducing the symptoms. The treatment usually consists of a period of exposure in the morning before dawn, and a period in the evening after sunset. The exact cause of the condition is unknown.