Light therapy or phototherapy consists of exposure to specific ranges of light wavelengths (using lasers or LEDs), or very bright, full-spectrum light, for a prescribed amount of time. It has proven effective in treating seasonal affective disorder (SAD), and for some people it has ameliorated delayed sleep phase syndrome. Proponents claim demonstrable benefits for skin conditions such as psoriasis, as well as more controversially, some degree of "skin rejuvenation".

Full sunlight is preferred for seasonal affective disorder. Other treatments are based upon infrared light exposure. There are a number of products (such as light boxes) using very intense artificial illumination that are effective for SAD. These products must provide 10,000 lux or more at the user's eyes, while filtering out any harmful ultraviolet radiation.

When treating delayed sleep phase, the timing of the exposure is critical. The light must be provided as soon after arising as possible to achieve any effect. Some users have reported success with lights that turn on shortly before waking.

Phototherapy is also used to treat neonatal jaundice, because light aids the breakdown of bilirubin in the skin.