Light Me up? Why, When, and How Much Light We Need

Light Anchors Us to the 24-H day
A circadian timing system, which is a set of clocks throughout the body that keeps daily time, controls many aspects of our physiology, metabolism, and behavior. It controls many internal biological rhythms, including body temperature, hormones, alertness and performance, and metabolic processes. These rhythms are generated spontaneously and have a natural rhythm near to—but not exactly—24 h. The circadian system also influences our behavior, such as the time when we can fall asleep and when we wake up naturally. The 24-h light and dark pattern, detected by the eye, is the key environmental time cue that resets, or synchronizes, the circadian system to the external, 24-h day. An example of this resetting process is observed when the light-dark cycle changes suddenly, as can happen when traveling to a new time zone or during shift work, when the circadian system little by little adapts to the new light-dark cycle. Our circadian system can typically shift up to 1 h each day without additional intervention. Light is the brain’s way of telling time.