Light Pollution, Circadian Photoreception, and Melatonin in Vertebrates

Abstract

Artificial light at night (ALAN) is increasing exponentially worldwide, accelerated by the transition to new efficient lighting technologies. However, ALAN and resulting light pollution can cause unintended physiological consequences. In vertebrates, production of melatonin—the “hormone of darkness” and a key player in circadian regulation—can be suppressed by ALAN. In this paper, we provide an overview of research on melatonin and ALAN in vertebrates. We discuss how ALAN disrupts natural photic environments, its effect on melatonin and circadian rhythms, and different photoreceptor systems across vertebrate taxa. We then present the results of a systematic review in which we identified studies on melatonin under typical light-polluted conditions in fishes, amphibians, reptiles, birds, and mammals, including humans. Melatonin is suppressed by extremely low light intensities in many vertebrates, ranging from 0.01–0.03 lx for fishes and rodents to 6 lx for sensitive humans. Even lower, wavelength-dependent intensities are implied by some studies and require rigorous testing in ecological contexts. In many studies, melatonin suppression occurs at the minimum light levels tested, and, in better-studied groups, melatonin suppression is reported to occur at lower light levels. We identify major research gaps and conclude that, for most groups, crucial information is lacking. No studies were identified for amphibians and reptiles and long-term impacts of low-level ALAN exposure are unknown. Given the high sensitivity of vertebrate melatonin production to ALAN and the paucity of available information, it is crucial to research impacts of ALAN further in order to inform effective mitigation strategies for human health and the wellbeing and fitness of vertebrates in natural ecosystems.

Working with Inadequate Tools: Legislative Shortcomings in Protection against Ecological Effects of Artificial Light at Night

“…Effects of disturbances caused by ALAN are manifold and affect a broad spectrum of species (Table 2). An omnipresent disturbance of various taxa is the suppression of circadian or seasonal rhythms when the natural signal of light is disguised by ALAN and hormone performance is affected [7].…”

“…If the spectrum is limited, the question arises, which colors cause the least impact on flora, fauna, and humans and should therefore be preferred? Organisms respond highly variable and sometimes contrary to different color spectra [7,12]. Thus, optimal light color conditions for all applications cannot be generalized.…”

“…Hence, outdoor lighting should have CCTs of 3000 K or less and if the installation is within or in vicinity of protected areas preferably less than 2400 K. Light-sensitive areas require the use of either low-or high-pressure sodium vapor lamps with light intensity regulation or LED with the lowest possible blue light component, such as narrow-band amber or phosphor converted (PC) amber LED. The illumination of freshwater systems, where the spectral composition of the water is altered by additional optically active constituents that absorb or scatter light, might be an exception because all colors seem to be detrimental [7,109,110].…”

“…So far, no conclusive thresholds of light intensities exist below which artificial light is not harmful to species and habitats. Laboratory studies indicate that even skyglow levels can affect organisms (e.g., references [7,326]). However, in outdoor field studies the light levels indicating effects seem to be much higher [87,94,109].…”

“…Nocturnal animals adapted their sensory systems to the nocturnal low-light conditions. Therefore, they can be directly affected if the low-light conditions are altered by ALAN, but diurnal animals are also known to be affected [7,8]. The duration and intensity of perceived daylight, the course of twilight, and the natural light during night time, in particular moonlight provide signals for orientation and rhythms and thus represent important information for most organisms.…”