Over the past decade, the night sky has brightened by an average of nearly 10 percent per year. According to projections, by 2052, half of the Earth’s land area will be affected by light pollution. Since large cities are concentrated near important natural areas such as coastal regions and freshwater bodies, and light pollution is reaching even seemingly untouched habitats, the natural day-night rhythm has changed for many animals and plants. “The light-dark cycle is one of the most stable environmental signals throughout evolution. A change in this signal due to light pollution therefore has far-reaching consequences for biodiversity,” said Dr Ashton Dickerson, the article’s lead author.

It is well known that the release of melatonin in all vertebrate groups – that is, birds, mammals, amphibians, reptiles, and fish – is suppressed by light pollution. On average, by 36 percent.

Plants are also affected by light pollution, as their seasonal rhythms are shifted: urban trees bud earlier and lose their leaves later. Research shows that light pollution influences the lengthening of the urban growing season more strongly than air temperature – especially toward the end of the growing season. Other documented consequences include changes in animal behavior throughout the day-night cycle. For example, some bird species begin singing earlier in the day.

Light pollution also alters the spatial distribution of animals by attracting or driving them away. Insects, fish, birds, and mammals are all affected. Flying insects are the most obvious example; the nocturnal swarms that gather around streetlights are well known. Numerous studies document this increase in the density of flying insects and their predators, e.g. spiders, near streetlights. Predatory reef fish also become more concentrated around illuminated corals, altering community composition. Other vertebrates often exhibit the opposite pattern: pumas, for example, avoid brightly lit habitats, as do migratory birds. Shorebirds show altered distribution patterns on nesting beaches, and eels veer off their migration routes near illuminated bridges. “Taken together, these responses lead to a redistribution of species into aggregation hotspots and alternative habitats. This results in the formation of novel communities with no historical precedent,” said Dr Franz Hölker, co-author of the article and head of the IGB research group “Light Pollution and Ecophysiology.”

According to the authors, there is still a need for better integration of ecological findings into lighting regulations and land-use planning. “There is still a gap between scientific knowledge and the protection of biodiversity,” summarized Franz Hölker. Some policy implementations already recognize light pollution as a threat to biodiversity. For example, the “Convention on Migratory Species” adopted the “International Light Pollution Guidelines” at COP 14 in 2024; Australia implemented the “National Light Pollution Guidelines for Wildlife” in 2023; and Germany introduced the so-called “Insect Protection Act” in 2021 (Section 41a BNatSchG).

Based on scientific findings regarding various species and contexts, the authors propose a four-tiered, hierarchical mitigation approach. In the first tier, light sources are permanently removed unless there is a justified need for them. In the second tier, spatial confinement is achieved with custom optics or shielding and luminous flux is reduced via dimming. The third level addresses temporal restrictions, such as limiting operating hours using timers and motion sensors. Finally, the fourth level involves spectral adjustment.

Major obstacles to effective regulation relate to proving relevant environmental damage and establishing a causal link: Most applicable environmental laws require, on the one hand, proof that the light emission in question causes a relevant impact and, on the other hand, that the light emission and the responsible operator can be identified. “Since it is often difficult to establish concrete environmental damage and identify those responsible, lawmakers could introduce evidence-based emission limits tailored to the specific type of lighting and its context of use,” said Dr Andreas Jechow, a professor at Brandenburg University of Applied Sciences and co-author of the study.

This solution would circumvent questions regarding the relevant harm and the causal relationship and establish clear, enforceable obligations. In particular, emission limits can be considered by manufacturers, retailers, and lighting designers, thereby reducing light pollution even before the lighting is installed.

In addition to emission control, planning tools and spatial protection concepts offer specific ways to ensure improved protection against light pollution. One approach, for example, is the creation of “dark corridors” to connect important biotopes – an approach that complements existing frameworks for habitat connectivity, such as Natura 2000.

The authors feel it is important to emphasize that this is not about neglecting the benefits of artificial light for people—above all, the sense of security it provides in the dark. New lighting concepts, such as those already implemented in some municipalities such as Fulda (Germany) or along Florida’s Gulf Coast (USA), show that ecological and societal requirements for public lighting can be reconciled.