Melatonin is an endogenously produced hormone that is involved in the regulation of the circadian rhythms of animals. Evidence suggests that the circadian rhythms of depressed individuals are disrupted, and early wakening and insomnia are a common symptom of depression. For this reason, researchers have been interested in the role melatonin may play in depression through the regulation of the circadian cycle. To these ends a number of studies have investigated the effects of exogenous melatonin on the symptoms of depression in depressed individuals. For example, in a large scale meta-analysis, researchers assessed a large body of literature containing a clinical research trials involving the use of melatonin supplements on the depressive symptoms of depressed individuals. The results of the study however showed that there was no significant effect for melatonin supplements on the depressive symptoms of patients with depression, as measured through self-rating and clinical-administered questionnaires.
Melatonin may have particular therapeutic effects against winter depression (seasonal affected disorder; SAD). The phase shift hypothesis suggests that individuals get depressed in winter months because of a delay in circadian rhythms relative to the sleep wake cycle. Administration of 0.125 mg of melatonin twice, 4 hours apart in the afternoon significantly improved the depressive symptoms of a group of individuals. This may be because melatonin administration in the afternoon causes a similar phase shift to bright early morning light, as occurs in the summer. Early morning melatonin production in patients with winter depression may cause a soporific effect that shifts the sleep wake cycle out of synchronicity with other circadian rhythms, and this may be what causes the depressive symptoms. Bright light therapy in the morning may have the same effect as exposure to early morning natural light in realigning the sleep wake cycle to the normal circadian rhythms.
So as a general therapeutic agent against depression melatonin may be limited in its effect. However, melatonin may be useful against a particular type of depression called winter depression (also called seasonal affective disorder; SAD). Winter depression is a seasonally produced set of depressive symptoms that likely result from the changing light conditions between winter and summer months in high latitude regions. Early morning light appears to have a greater antidepressant effect that evening light, and exposure to this light in summer months can reset circadian rhythms by causing a phase advance. In the winter, the lack of morning light causes a phase shift in circadian rhythms and this might lead to sleep disturbance and depression. Melatonin supplements in the afternoon may have the same phase shifting effect. Studies suggest that melatonin at a dose of 0.125 mg 4 hours apart in the afternoon does indeed relieve the symptoms of SAD, possibly through a phase shifting effect.
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