Lighting up the nighttime | Science

![Figure][1]The bright lights of Tokyo and other cities in Japan, photographed from the International Space Station in 2015, show the proliferation of artificial light on our planet.PHOTO: NASA, SCOTT KELLYAmong the most visually compelling images of the whole Earth have been those created using data obtained at night by astronauts or from satellites. The proliferation in use of electric lighting—including from industrial, commercial, municipal, and domestic sources—is striking. It sketches the spatial distribution of much of the human population, outlining a substantial proportion of the world’s coastline, highlighting a multitude of towns and cities, and drawing the major highways that connect them. The data embodied in these nighttime images have been used to estimate and map levels of energy use, urbanization, and economic activity. They have also been key in focusing attention on the environmental impacts of the artificial light at night itself. Explicit steps need to be taken to limit these impacts, which vary according to the intensity, spectrum, spatial extent, and temporal dynamics of this lighting.Artificial light at night can usefully be thought of as having two linked components. The first component—direct emissions from outdoor lighting sources, which include streetlights, building and infrastructure lighting, and road vehicle headlamps—is spatially extremely heterogeneous. Ground-level illuminance in the immediate vicinity can vary from less than 10 lux (lx) to more than 100 lx (for context, a full moon on a clear night has an illuminance of up to 0.1 lx). It often declines rapidly over distances of a few meters. However, emissions from unshielded lights can, when unobstructed, carry horizontally over many kilometers, making artificial light at night both an urban and a rural issue.The second component of artificial light at night is skyglow, the brightening of the nighttime sky caused mainly by upwardly emitted and reflected artificial light that is scattered in the atmosphere by water, dust, and gas molecules. Although absolute illuminance levels are at most about 0.2 to 0.5 lx, much lower than those from direct emissions, these are often sufficiently high to obscure the Milky Way, which is used for orientation by some organisms. In many urban areas, skyglow even obscures lunar light cycles, which are used by many organisms as cues for biological activity.In the laboratory, organismal responses, such as suppression of melatonin levels and changes to behavioral activity patterns, generally increase with greater intensities of artificial light at night. It is challenging to establish the form of such functional relationships in the field, but experiments and observations have shown that commonplace levels of artificial light at night influence a wide range of biological phenomena across a wide diversity of taxa, including individual physiology and behavior, species abundances and distributions, community structure and dynamics, and ecosystem function and process ([ 1 ][2]). Exposure to even dim nighttime lighting (below 1 lx) can drastically change activity patterns of both naturally day-active and night-active species. These effects can be exacerbated by trophic interactions, such that the abundances of species whose activity is not directly altered may nonetheless be severely affected under low levels of nighttime lighting ([ 2 ][3]).Globally, the prevailing technology of outdoor lighting is undergoing a marked shift to light-emitting diodes (LEDs). Although LEDs can be used to produce a wide diversity of emission spectra, the main trend has been for narrower-spectrum street lighting with lower (“warmer”) correlated color temperature (CCT) to be replaced by broader-spectrum LED lamps with higher (“cool white”) CCT. This lighting provides improved color rendering, more faithfully revealing colors as seen under sunlight, but also tends to exacerbate skyglow unless accompanied by dimming and improved shielding ([ 3 ][4]). Biological responses to light are almost invariably spectrum-dependent, and broadening the spectrum of emissions increases the likelihood of their overlapping with these patterns of sensitivity, often increasing the biological impact.Of particular concern is the growth in emissions of blue wavelengths, to which melatonin suppression is disproportionately sensitive. Multiple cascading processes can include stress responses, disease risk, and likelihood of obesity. Medical organizations have advised that poorly designed high-intensity and high-CCT street lighting should be avoided to minimize potential harm to human sleep patterns, sleep quality, and circadian rhythms ([ 4 ][5]). Studies have also raised concerns that greater exposure to artificial light at night increases some cancer risks ([ 5 ][6]). However, it is difficult to isolate effects of outdoor lighting from those of indoor lighting (including the trespass of outdoor lighting indoors) and to adequately control

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