Seasonal affective disorders (SAD) are a type of mood disorder, either unipolar or bipolar, in which there has been a correlation between the appearance or disappearance of episodes and a particular time of the year. In other words, they are episodes of mood disorders brought on by fluctuations in the seasons. Although it is not clear exactly how it works, light therapy (exposing the patient to a bright source of light on a daily basis) has been proven to be an effective form of therapy for this disorder; improvements are seen within two to five days.