Circadian rhythms are the daily oscillations in our physiology and behavior. Ideally, the 24-hour, light/dark pattern on the retina synchronizes these daily oscillations to our local position on Earth. Retinal light exposure affects a wide range of neural channels to the brain, one of which is the pathway to the suprachiasmatic nuclei (SCN) in the brain, the biological clock that orchestrates circadian rhythms. Consequently, the definition of light as the stimulus for the human circadian rhythms must be different from the definition of light as the stimulus for vision. Constrained by retinal neurophysiology, a spectral sensitivity model for the human circadian system was developed. Among the model predictions are the human circadian system’s high threshold and subadditive responses to retinal light exposure. This model has been useful for predicting the threshold and suprathreshold response of the SCN to retinal light exposure.

Mark S. Rea, Ph.D., is a Professor in the Department of Department of Population Health Science and Policy at the Icahn School of Medicine at Mount Sinai. He was formerly a Professor of Architecture and Cognitive Sciences at the Lighting Research Center (LRC) at Rensselaer Polytechnic Institute and served as LRC Director from 1988 to 2017. Dr. Rea is well known for his research in circadian photobiology, mesopic vision, psychological responses to light, lighting engineering, and visual performance. He is the author of more than 250 scientific and technical articles related to vision, lighting engineering, and human factors and was editor-in-chief of the 8th and 9th editions of The Lighting Handbook (Illuminating Engineering Society [IES] of North America, 1993, 2000). He has been elected Fellow of the Society of Light and Lighting (UK) and Fellow of the Illuminating Engineering Society and is a recipient of the IES Medal. While at Rensselaer, Rea received the William H. Wiley Distinguished Faculty Award, which honors those who have won the faculty’s respect for excellence in teaching, productive research, and interest in the totality of the educational process. Dedicated to the notion that our society undervalues light because we do not properly measure its benefits, his book Value Metrics for Better Lighting brings together a wide range of research to illustrate how the effective use of light can benefit society and the environment.