Without light, there would be no life on Earth. Electromagnetic radiation (EMR) across a very wide spectrum heats the planet sufficiently for biological activity; EMR across the range from ~380 to ~780 nm is responsible for most plant life and, most importantly to us, stimulates photoreceptive cells in the retina of most creatures with eyes. This is so important to humans that the very definition of light in the Système International d’Unités [the International System of Units] is based on our biological response to this part of the electromagnetic spectrum (Commission Internationale de l’Eclairage [CIE], 2011), the only physical quantity that is linked to a human capability. This photodetection reveals the world to us through vision and attunes us to the cyclical pattern of light and dark so that we – as virtually all life on earth (fauna and flora) – have periods of activity and periods of rest. It is hard to overestimate the role light has in all aspects of human function, and particularly those of interest to psychologists: perception, cognition, affect, communication, comfort, sleep.