Light, opsins, and life: Mammalian photophysiological functions beyond image perception

Light, a fundamental form of energy and sensory input, has significantly shaped life forms on Earth. In mammals, light perception through the eyes, which enables image formation, is crucial for survival. However, beyond image-forming (IF) vision, light also mediates non-image-forming (NIF) functions, such as circadian photoentrainment and the pupillary light reflex. Recent studies have further demonstrated that light influences a wide range of physiological and behavioral processes, including mood, metabolism, cognition, pain perception, sleep, and neuronal development. The diverse types of opsins, the major photosensitive proteins in mammals, are expressed not only in the rods, cones, and intrinsically photosensitive retinal ganglion cells (ipRGCs) of the retina but also in extraocular tissues, such as the brain, skin, and adipose tissue. Opsins in both ocular and extraocular tissues jointly contribute to light detection and mediate diverse NIF functions. In this review, we focus on the NIF effects of light on mammals, emphasizing its regulation of physiological functions as well as the corresponding roles of light receptors and associated neuronal circuits. It also highlights the implications of these findings for human health, underscoring the need for a comprehensive understanding of the interactions between light and life.