Seasonal affective disorder (SAD) is a disturbance of mood and behavior which resembles some seasonal changes seen in lower mammals. Like these animal seasonal changes, SAD is thought to be related to decreased sunlight during winter months. [SAD has been successfully treated with exposure to bright artificial light of higher intensity than is usually present in the home or workplace. Many people not suffering from SAD may nonetheless have seasonal changes which could be helped by environmental light supplementation. Lighting standards in the home and workplace should be re-evaluated on the basis of new knowledge of the psychobiological effects of light.] We review the literature on SAD and discuss its public health implications in the context of a typical case presentation.