Light therapy is a way to treat seasonal affective disorder (SAD) by exposure to artificial light. Seasonal affective disorder is a type of depression that occurs at a certain time each year, usually in the fall or winter.During light therapy, you sit or work near a device called a light therapy box. The box gives off bright light that mimics natural outdoor light.Light therapy is thought to affect brain chemicals linked to mood, easing SAD symptoms. Using a light therapy box may also help with other types of depression, sleep disorders and other conditions. Light therapy is also known as bright light therapy or phototherapy.