MINI REVIEW The dark side of light at night: physiological, epidemiological, and ecological consequences

Successful organisms must adapt to temporal, as well as spatial niches. Endogenous biological clocks allow individuals to anticipate and adapt to the daily light-dark cycles in their environments to optimally time metabolism, physiology, and behavior each day. Rodents in nontropical environments, for example, alter reproductive, metabolic [1], and immunological activities [2] based on changes in day length throughout the seasons. The timing of avian reproduction and molt also often depends upon seasonal changes in day length [3], and many species, including some birds [4,5], rodents [6], bats [7], and marine animals [8], adjust foraging activities according to changes in the lunar cycle. Aside from seasonal adjustments, there is marked circadian variation in physiological functions. In many species, including some birds, rodents, fish, and humans, for example, circulating concentrations of sex steroids [9–11] and glucocorticoids [12] vary with the light/dark cycle throughout the day, causing corresponding changes in reproductive activities [13] and metabolic functions [14]. Responses to natural light cycles result in an adaptive temporal organization in humans and other animals. With the invention and use of electrical lights, beginning about a century ago, this temporal organization has been dramatically altered. Light at night has significant social, ecological, behavioral, and health consequences that are only now becoming apparent. The extensive control that light-driven mediators exert upon multiple body systems, for example, creates numerous targets on which light-induced disruptions can act, resulting in a wide range of physiological changes and potentially serious medical implications. In a broader context, underpinning physiological mechanisms regulate a variety of behaviors, ranging from reproduction to foraging, creating expansive targets for light disruption. Assuming that adaptive processes have optimized the physiological and behavioral regulation of animals according to changing day lengths and circadian cycles, artificial changes in light cycles could have drastic fitness effects. This review summarizes the medical and ecological implications of exposure to artificial light at night, and related disturbances in normal seasonal and circadian physiological and behavioral functions.