light therapy light therapy light therapy, also called phototherapy or bright light therapy is the exposure to direct sunlight or artificial light at controlled wavelengths in order to treat a variety of medical disorders, including seasonal affective disorder (sad), circadian rhythm sleep-wake disorders, cancers, and skin wound infections. treating skin conditions such as neurodermatitis, psoriasis, acne vulgaris, and eczema with ultraviolet light is called ultraviolet light therapy.