<qa>  What is the most powerful signal from the environment that influences and regulates daily biological rhythms?Light is the most powerful signal from the environment that influences and regulates daily biological rhythms. It is well-established that the irradiance, duration, and timing of light exposure all affect the response of the circadian system. While it was once thought that these responses were mediated through the visual system, it is now known that there is a network of intrinsically photosensitive retinal ganglion cells ipRGCs that mediate circadian responses to light. Other new studies have demonstrated that recent light exposure history affects the circadian response to light in humans. 