Toward a Biophysical Definition of Light for the Human Circadian System Dr Mark Rea

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.