The biological impacts of artificial light at night: the research challenge.

One contribution of 14 to a theme issue 'The biological impacts of artificial light at night: from molecules to communities'. 1. The challenge Daily, lunar and seasonal cycles of natural light have been key forms of environmental variation across the Earth's surface since the first emergence of life. They have driven the development of biological phenomena from the molecule to the ecosystem, including metabolic and physiological pathways, the behaviour of individuals, geographical patterns of adaptation and species richness, and ecosystem cycles (e.g. [1–4]). Indeed, biological systems are arguably organized foremost by light [5– 7]. The natural patterns of light have over the last 100 years come to be greatly disrupted through the introduction of artificial light into the night-time environment: artificial light at night (ALAN). This derives from a diversity of sources, including street lighting, advertising lighting, architectural lighting, security lighting, domestic lighting and vehicle lighting. ALAN disrupts natural patterns of light both via direct effects of illumination from these sources as well as via skyglow (the scattering by atmospheric molecules or aerosols in the atmosphere of ALAN that is emitted or reflected upwards; [8–10]). On the ground this disruption of natural patterns of light takes two principal forms [11]. First, light has been introduced in places, times and at intensities at which it does not naturally occur. This has been firmly fixed in the public imagination through the creation from satellite and astronaut acquired night-time imagery of pictures of the Earth which illustrate the extent of ALAN, of urbanization and of major centres of human population (figure 1; e.g. [12,13]). Given the nature of such images, it is challenging to use these to make some categorical quantification of the extent of ALAN, although it is plainly much more widespread than urban infrastructure alone, mainly because of the skyglow effect. One estimate that accounted explicitly for the effects of skyglow was that 18.7% of the global land area experienced ALAN [12], another based more directly on satellite imagery that 11.4% of terrestrial and 0.2% of marine areas of the globe experienced ALAN [11], and another that ALAN is increasing at around 6% per annum with huge geographical variation (0–20%; [14]). Second, ALAN is introducing light with a spectrum that is different from those of sunlight, moonlight or starlight [11]. The spectrum of ALAN depends fundamentally on the kind of lighting device that is being used, ranging from narrow (e.g. low …