Light therapy (LT) has become increasingly popular in various countries around the world in the last decade. For instance, according to a recent survey carried out in Germany in 1992, 13% (n = 56) of all German psychiatric hospitals (n = 422) used LT for different treatment indications and another 8% indicated their interest to do so. Among university facilities LT is even more popular, with a percentage of 57%. Although the most frequently used treatment indication for LT is seasonal affective disorder (SAD) or its subsyndromal form (S-SAD) it is apparent that other forms of depression e.g. non-seasonal forms, either acute or chronic are also a target for this new treatment modality. There is a number of studies supporting the use of LT for SAD (for review), however there are just a few studies for non-seasonal depression or for the other treatment indications (for review).

Melatonin is a hormone secreted by the pineal gland mainly during the night. The discovery that this melatonin secretion decreases under the influence of bright light, gave rise to the use of light therapy in some affective disorders. The literature on the relationship between melatonin secretion and mood is reviewed concerning seasonal affective disorder, non-seasonal affective disorder and premenstrual syndrome. Light therapy could reduce an abnormal high melatonin secretion back to normal proportions. None of the affective disorders, however, is accompanied by an unusual high melatonin level. Nevertheless, light therapy as well as other therapies that suppress melatonin have a therapeutic effect. This is not the case with the administration of melatonin. Mood is not affected by extra melatonin in seasonal affective disorder but it is in both other affective disorders. Melatonin plays a part in the pathogenesis of the affective disorders but it is not yet clear which one.