Minimal Mimics and Maps of Natural Light for Mammals

Abstract

Light drives processes that include perception and the regulation of circadian rhythms, sleep, metabolism, and development. These processes are initiated by photopigment molecules, each preferentially absorbing particular wavelengths. Light of a given spectrum stimulates an animal's set of photopigments in a specific profile. Natural skylight and its variations produce stimulation profiles that promote normal physiology. To mimic these profiles using artificial light, we consider the thermally stable, photoconvertible states of relevant photopigments: ground states of rhodopsin and cone photopigments, and three states of melanopsin. This gives a relatively high-dimensional representation of illumination. Nevertheless, we find that two wavelengths suffice to closely mimic the effects of natural light for mammals, including humans and mice. Adjusting the wavelength ratio allows mimicry of natural light's variations, such as those from twilight to noon. Ratio adjustments also compensate for light's filtering by elements like the eye's optics and laboratory cages. Adding a third wavelength makes natural light mimicry nearly perfect. By contrast, common artificial lighting—designed for low-dimensional, human color space—stimulates photopigments in unnatural proportions. We conclude by providing species-specific maps of photopigment stimulation profiles under natural and artificial illumination, which make our observations intuitive while providing insight into the diverse visual ecologies of mammals. These mimics and maps concern either sex.