Human melatonin suppression by light: a case for scotopic efficiency.

Human adult males were exposed to combinations of two illuminances and two spectral power distributions over the course of four nighttime sessions. A dose-dependent response of acute melatonin suppression to light was found, but photopic (cone-based) illuminance did not adequately predict suppression. When melatonin suppression was plotted against scotopic (rod-based) illuminance, the data formed a nearly monotonic function, implicating rods, or a rod-dominated mechanism, in the human melatonin regulation system. The results do not, however, rule out mechanisms other than rods, including novel photoreceptors, as candidates for melatonin regulation in humans.