People who suffer from SAD are affected by the amount of daylight exposure they receive. The hormone melatonin, which helps regulate hormone secretion, body temperature and sleep, is produced in the brain during hours of darkness. The decrease in daylight hours in autumn and winter causes an increased production of melatonin in SAD sufferers, which results in the onset of debilitating symptoms of depression. Exposure to bright light is effective in the treatment of seasonal affective disorder.

The incidence of SAD as well as its severity is influenced by several factors including geographical location, family history of SAD, recent illness or spending a major portion of the day in a windowless room. Light therapy works on the principle that exposure to bright light can suppress the production of melatonin in the brain, which regulates the internal clock of the body and reduces symptoms.

A light box is a small portable device that is comprised of fluorescent tubes or bulbs. It is specially designed to provide a precise amount of balanced spectrum light that is equivalent to being out in the sun on a clear, sunny spring day. In mimicking outdoor light, the light box causes biochemical changes in the brain that help in synchronizing wake/sleep patterns, regulating the body clock and relieving symptoms of SAD.

During Light Box Therapy the user is required to sit directly in front of the light box at a specific distance, which depends upon the required intensity. For light therapy to be effective it is necessary for the light from the light box to enter the eyes indirectly. Just exposing the skin to the light does not have the same effect. Looking directly into the light can damage the eyes and is not advisable, except in a few patients who are asked to look into the light for brief moments before looking away again. However, sunglasses are not to be worn during light therapy as it blocks the light from entering the eye, adversely affecting the effectiveness of the therapy.

Light therapy is now considered a standard treatment for SAD. It offers numerous potential benefits for SAD sufferers especially for those who are averse to taking antidepressants or for those tried antidepressants and found them ineffective.