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 suppressio n. When melatonin suppression was plotted against scotopic (rod-based) illu minance, the data formed a nearly monotonic function, implicating rods, or a rod-dominated mechanism, in the human melatonin regulation system. The re sults do not, however, rule out mechanisms other than rods, including novel photoreceptors, as candidates for melatonin regulation in humans. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.