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. SIGNIFICANCE STATEMENT Humans sense light for vital processes like sight and physiological regulation. These processes are normal under natural skylight, where they evolved. However, much of modern life is spent in artificial light, which is unlike natural light in many of its biological effects. This disparity has been linked to disorders that range from cardiovascular disease to cancer. This manuscript introduces simple forms of artificial lighting that replicate the effects of natural skylight on photoreceptors of humans and other species. It also demonstrates how the biological effects of natural and artificial lights can be captured in simple maps, facilitating the choice and further design of illumination that is beneficial. Competing Interest Statement Philippe Morquette and Michael Tri H. Do declare that a patent application related to the work described in this manuscript has been filed through Boston Children's Hospital. Footnotes In this version of the manuscript, some parts have been rearranged and edited for clarity; additional references have been added; and small corrections to the figures and text have been made.