Melatonin decreases the amplitude of the b-wave of the human electroretinogram.

In a double-blind placebo crossover study of 13 healthy volunteers, the pineal hormone melatonin (10 mg) was given at 4 pm, and the electroretinogram measured under conditions of dark and light adaptation. A significant diminution of b-wave amplitude was found under both photopic (delta = 5.4 microV, p < 0.05) and scotopic conditions (delta = 7.4 microV, p < 0.01). These data indicate that melatonin may transduce the dark signal at the level of the retina as well as the pineal. Acute administration of melatonin decreases sensitivity to light.