bats and lighting
Dr Stone started working on bats and lighting in 2005 when she started her PhD research at the University of Bristol. During this time she developed and conducted novel field research experiments, taking lighting to bats in the field to assess impacts on foraging and commuting behaviour, with a focus on lesser horseshoe bats (Rhinolophus hipposideros) (you can find out more about Lesser horseshoe bats here).
Using the results from her PhD with funding from Natural England Dr Stone created a working group to bring together stakeholders in the UK and published the first UK based guidance for mitigating the impacts of lighting on bats.
We have been working to understand the impacts of artificial lighting at night (ALAN) on bats for over 20 years.
This Guidance is updated annually by the Bat Conservation Trust, and Dr Stone is now working with Eurobats to contribute to the European Bat Mitigation Guidance due out in Nov 2026.
Our early work focused on the Lesser horseshoe bat (R. hipposideros) however over the years we have worked with many species to assess the impacts of lighting including Myotis nattereri, Myotis daubentonii and Pipistrellus bats.
We have now tested the impacts of various different light types, light levels and in different environments including roosts, tunnels, churches and waterways. See our publications page for our outputs and research thus far.
Find out more about the impacts of lighting on bats below or browse our projects to see what we are currently researching in this field.
bats and lighting
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 [5]. Evidence indicates, however, that ecological impacts of artificial lighting are vast, and require urgent attention [4].
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 [8]. 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 [9].
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 [11]. Approximately half of Europe and a quarter of North America now experience a disrupted day/night cycle due to ALAN [11]. 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 [13].
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 [7]. Slow-flying species of bat such as Myotis spp. and Rhinolophus hipposideros tend to emerge from their roosts some time after nightfall to avoid predators such as peregrine falcons [5] and therefore are believed to prefer naturally darker environments. The light type and colour, habitat and bat activity also influences the responces of bats to light [14]
Some bats may be more tolerant to artificial lighting
Faster flying bats such as Pipistrellus spp. emerge before sunset [5, 7], and have been recorded foraging at light sources as a result of the increased densities of insects attracted to some light types (Fig. 4) [17]. Our research has shown that tympanate moths (those with "ears" that can hear bat echolocation and take evasive action) are attracted to light sources and show reduce evasive behaviours under white light, making them easier prey for bats. This reduced predatory avoidance may cause competitive exclusion for those bats which tend to prefer 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 light, fewer feeding buzzes were heard at these lights, suggesting the bats were not feeding as would be expected [7].
Some bats avoid artificial lighting
We conducted experiments in the South-West of England along 10 hedgerows and measured the effect of three different light intensities using LED lights [2]. We found that Myotis spp. activity was lower at all intensities compared to no light (Fig. 5). Rhinolophus hipposideros activity however, was sequentially lower with higher intensities. Both species were observed to actively avoid light, switching to the unlit side of hedges; therefore, use of artificial lighting may result in interference with migration navigation and reduction in fitness as bats may be forced to travel further to forage [7].
Despite the apparent tolerance of Pipistrellus spp. to light, dark corridors appear to remain the preference for commuting [19]. 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 [20]. This delay in emergence results in fewer feeding opportunities and reductions in fitness [2].
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, impacting movement patterns, roosting opportunities, and potentially 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 [17]. Even smaller adaptable 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.