The discovery of a novel photoreceptor in the retina that participates in circadian phototransduction (how the retina converts light signals into neural signals) has opened up a new area of investigation. It is still not known whether human circadian phototransduction uses mechanisms similar to color vision, i.e., opponent mechanisms at a post-receptoral level. The primary goal of the study reporter here was to answer the question: does the circadian system respond to light in an additive manner, or is a spectral opponent mechanism also involved? A secondary goal of this work was to determine if the spectral sensitivity of nocturnal melatonin changes throughout the night. The findings in this book are a first step toward a better understanding of the mechanisms underlying circadian phototransduction in humans. The present results demonstrate that an opponent mechanism contributes to the spectral sensitivity to light for human melatonin suppression. In general, retinal stimulation of humans from monochromatic, or nearly monochromatic, light sources might result in data that cannot be generalized to polychromatic sources used in architectural lighting.