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.
