Circadian Potency Spectrum in Light-Adapted Humans

Light exposure at night can disrupt the circadian timing of cellular processes and is associated with a broad range of health disorders. To spectrally engineer lighting which minimizes circadian disruption at night it is necessary to define the precise spectral sensitivity of the human circadian system. Prior attempts have used short monochromatic light exposures in dark-adapted human subjects, or in vitro dark-adapted isolated retina or melanopsin. However, humans spend virtually all their awake hours in a fully light-adapted state. Here we review the evidence for a narrow blue circadian sensitivity curve for light-adapted humans derived from experiments using spectral filtering of light sources, and comparisons of light sources with diverse spectral power distributions. This light-adapted Circadian Potency function permits the development of circadian-protective light for nocturnal use and circadian-entraining light for daytime use.