The action spectrum for percent control-adjusted melatonin suppression in 72 healthy human subjects. The filled circles represent the half-saturation constants of nine wavelengths from 420 to 600 nm that were normalized to the maximum response and plotted as log relative sensitivity. The solid curve portrays the best-fit template for vitamin A1 retinaldehyde photopigments, which predicts a maximal spectral absorbance (λmax) of 464 nm. There is a high coefficient of correlation for fitting this opsin template to the melatonin suppression data (R2 = 0.91). This figure and the data it represents elucidated the wavength sensitivity of a novel sensory system in the human eye and have been instrumental in developing new lighting technologies and lighting methodologies for light therapy and human health.