Light therapy, or phototherapy, is the use of bright full-spectrum phlorescent light – the closest form of light to sunlight-- for medicinal purposes. It has been used as a therapeutic modality for Seasonal Affective Disorder (SAD) since the late 1980s. Anyone who lives in a Northern latitude, where winter days are short, or spends long hours in a windowless environment, can receive insufficient natural light, but people with seasonal depression, suffer the loss acutely. Phototherapy consists of precisely timed exposure to white fluorescent light or full spectrum light in a light box. The lights are covered with a plastic screen to block ultraviolet rays. Effective treatment is exposure to 10,000 lux of light -- the amount of visible light impinging on a square metre surface-- for about a half hour every morning. In dawn simulation, exposure to artificial light may begin before awakening. Lower intensities of light, while also effective, require longer exposure. Dose of light and length of exposure are always prescribed and adjusted according to individual need and response.