One of the major causes of global loss of biodiversity is artificial light at night (ALAN). ALAN refers to the use of artificial lighting that alters natural night-time light levels [1, 2, 3, 4]. Consequences of ALAN include the well documented reduction in visibility of stars in urban environments, pictured above . Evidence indicates, however, that ecological impacts of artificial lighting are vast, and require urgent attention .
Light pollution affects the ecological interactions of a range of organisms by altering physiology, behaviour, reproduction and genetic fitness [4, 5, 7]
What is ALAN?
Globally ALAN produces an estimated 1900 Mt of CO2 annually, consuming 19% of electricity produced . Growing awareness of climate change and shifts in legislative policies have led to improvements in technology and efficiency, and less energy efficient traditional high and low pressure sodium bulbs are being replaced with broad spectrum light emitting diodes (LED) and ceramic metal halide lights [2,7]. LED lighting generally does not emit insect-attracting ultraviolet (UV) light, unlike metal halide lights and mercury vapour lights (Fig. 2), although LED lighting still attracts many invertebrates .
In the first half of the 20th century, ALAN increased worldwide by an average of 6% (range 2-20%); however, from 2012-2016 the global area impacted by ALAN increased by 2.2%, with radiance increasing at a similar rate . Approximately half of Europe and a quarter of North America now experience a disrupted day/night cycle due to ALAN . In the UK, over the last 50 years the energy efficiency of lighting has doubled, while the annual energy consumption for lighting has quadrupled [6, 12]. Only 46.2% of Britain still has pristine dark night skies, which equates to 21.7% in England, 56.9 in Wales, and 76.8% in Scotland .
How does ALAN affect bats?
Responses to artificial light in bats are species-specific (fig. 3), believed to be due to flight morphology and echolocation . Slow-flying species of bat such as Myotis spp. and Rhinolophus hipposideros tend to emerge longer after nightfall to avoid predators such as peregrine falcons  and so prefer to avoid lights. The light type and colour, habitat and bat activity also dictates the repsonse to light 
Some bats may be attracted to artificial lighting
Faster flying bats such as Pipistrellus spp. are better adapted in predator avoidance, often emerging before sunset [5, 7], and are more likely to be attracted to light sources as a result of insect light attraction (fig. 4) . Tympanate moths (moths that have evolved organs with which to hear bat echolocation and take evasive action) attracted to light sources have been shown to reduce evasive behaviour under white light, making them easier prey for bats, which may cause competitive exclusion and increased competition with light avoidant bats foraging in nearby dark areas [7, 9, 18]. However, a study led by Dr Emma Stone found that despite bat and invertebrate activity being higher at white metal halide light compared to orange, fewer feeding buzzes were heard at these lights, suggesting the bats were not feeding as would be expected .
Some bats avoid artificial lighting
A controlled study led by Dr Emma Stone in the South-West of England along 10 hedgerows measured the effect of three different light intensities using LED lights . Findings of this study (figure 3) indicate that Myotis spp. activity was lower at all intensities compared to no light. R. hipposideros activity; however, was sequentially lower with higher intensities. Both species were observed to actively avoid the light, choosing to use the unlit side of the hedge; therefore, use of artificial lighting may result in interference with winter migration navigation and reduction in fitness from needing to travel further to forage .
Despite the apparent attraction of Pipistrellus spp. to light, dark corridors appear to remain the preference for commuting . Lighting outside roosts may prevent bats from leaving; a study at two bat roosts in Aberdeenshire indicated that fewer bats left the roosts when they were illuminated by white or blue halogen light compared to being unlit . This delay in emergence results in fewer feeding opportunities and reduction in fitness .
Other impacts of ALAN
Because of light avoidance, installation of new, inappropriate, or poorly researched lighting creates habitat fragmentation, driving away less light-tolerant bats, which impacts migration patterns, roosting opportunities, and genetic fitness. Sky glow caused by light reflecting off clouds makes bats more vulnerable to predators and obscures sunset, disorientating bats emerging from their roosts . Even smaller light-tolerant bats such as Pipistrellus spp. are at risk of these consequences, and their increased foraging opportunities may in turn decrease the foraging opportunities of those who remain in the dark.