Phototherapy, or light therapy, is the administration of doses of bright light in order to normalize the body's internal clock and/or relieve depression. Phototherapy is prescribed primarily to treat seasonal affective disorder (SAD), a mood disorder characterized by depression in the winter months, and is occasionally employed to treat insomnia and jet lag.

The exact mechanisms by which the treatment works are not known, but the bright light employed in phototherapy may act to readjust the body's circadian (daily) rhythms, or internal clock. Other popular theories are that light triggers the production of serotonin, a neurotransmitter believed to be related to depressive disorders, or that it influences the body's production of melatonin, a hormone derived from serotonin that may be related to circadian rhythms.

Patients with eye problems should see an ophthalmologist regularly, both before and during phototherapy. Because some ultraviolet rays are emitted by the light...