Melatonin reference limits at 3:00 AM and 8:00 AM in healthy adults.

We determined melatonin reference limits in blood serum of 129 healthy volunteers, aged 21 to 70 years, using enzyme-linked immunosorbent assay (IBL ELISA kit, Hamburg, Germany). At 3:00 AM, in almost full dark, the reference limits were found to be 18.3-134 pg/ml in males, 19.