Blood serotonin, serum melatonin and light therapy in healthy subjects and in patients with nonseasonal depression.

The 24-h rhythms of blood serotonin and serum melatonin were determined in 39 unmediated inpatients with nonseasonal affective disorder and in 14 healthy men and women after 7 days of morning bright-light (2500 lx) or dim-light (50 lx) treatment. Bright-light treatment led to a more than 50% decrease in the Hamilton Rating Scale for Depression (HRSD) score in 4/19 patients and dim light in 1/17 patients. After light treatment the mesor (the daily mean estimated by cosinor analysis) of patients' and subjects' melatonin levels did not change significantly, nor was there a correlation between phase change and decrease in HRSD score. We observed after bright- and dim-light treatment a consistent increase in blood serotonin in patients and healthy subjects, which differed significantly between healthy subjects and patients. These findings suggest the involvement of serotonergic mechanisms following light therapy.