The chronosense -- what light tells man about biological time

In the past 10 years, experimental studies have provided further evidence for the suggestion that the eye serves man as a dual sense organ, viz as a sense organ for sight but also for time and the regulation of biological rhythms. A small group of scientists interested in the adjustment of biological rhythms to the key Zeitgeber light wanted to answer the question whether rods and/or cones and/or other uncharacterized retinal photoreceptors contribute to this function in mammals. Intriguingly, in the course of elegant research, a number of laboratories around the world have been zeroing in on a novel non-rod, non-cone ocular photopigment which serves a number of responses to non-image-forming (NIF) photoreception in mammals. This paper intends to draw attention to possible implications of photoreception and phototransduction research for other scientific disciplines which study health and diesase effects in man. We therefore review the pivotal role of the photoreceptors -- old and new -- for the light-related timing and coordination of the interplay of otherwise less efficient biological rhythms. To distinguish our focus on time- and timing-related effects from classic image-forming (IF) and other NIF responses to ambient light, we refer informatively to chronoreceptors which mediate the sense of time, or chronosense. We conclude that syndisciplinary research into the physiology and pathophysiological implications of the chronosense is warranted and summarize a series of research questions.