Light at Night: Infrastructure, Biology, and the Quiet Costs
by Mike Berst
February 26, 2026
Artificial light is one of the most taken-for-granted features of modern life. Streets are lit, parking lots glow, buildings shine through the night, and very few of us stop to ask what that light is doing beyond helping people see where they're going. Yet over the last several decades, it has become increasingly clear that artificial light at night is not a neutral convenience. It is a powerful biological signal, and like all such signals, it carries consequences.It is a powerful biological signal, and like all such signals, it carries consequences. For a practical overview of how outdoor lighting affects wildlife and what can be done about it, see our guide to artificial light problems.
A small but telling example comes from Gladsaxe, a municipality near Copenhagen, where planners have begun installing red-dominant LED streetlights along specific roadways. The project has attracted attention because it departs from the prevailing assumption that brighter, whiter light is always better. The stated motivation is modest and practical: to reduce the impact of streetlighting on local bat populations while maintaining adequate visibility for people. What makes the project significant is not its scale, but its premise. It treats light pollution as an infrastructure problem with an infrastructure solution, rather than as an unfortunate side effect to be managed by individual awareness or goodwill.
Bats are the most visible focus of the Gladsaxe experiment, and for good reason. Many bat species are highly sensitive to artificial light, particularly short-wavelength white and blue light. Such lighting can disrupt flight paths, fragment habitat, and interfere with feeding behavior. In landscapes where darkness once provided safe corridors between roosts and feeding areas, bright lighting can act as a barrier. Red-dominant lighting, by contrast, is far less disruptive to bat behavior—not because it is “gentler” in some abstract sense, but because it interacts differently with their sensory and physiological systems.
What is often missed in public discussions is that bats are not an isolated case. They are simply one of the easiest animals to notice when lighting changes alter behavior. The same light that disorients bats also affects insects, birds, and mammals through shared biological mechanisms.
Among insects, moths are especially vulnerable. Many species are strongly attracted to artificial light, where they circle endlessly, become easy prey, or die from exhaustion. More subtly, this attraction pulls them away from normal feeding and breeding behavior. Even when artificial light does not kill moths outright, it reduces the likelihood that they will mate successfully or lay eggs in appropriate habitat.
This matters because moths are not just insects; they are part of a much larger ecological chain. Moth larvae—caterpillars—are a primary food source for many nesting birds. During the breeding season, adult songbirds often rely almost entirely on soft-bodied larvae to feed their young.
The timing is precise. Birds initiate nesting to coincide with peaks in caterpillar availability, which in turn depends on the life cycles of moths and other insects. When artificial light suppresses moth reproduction or alters insect abundance, the effects may not be immediately visible, but they reappear weeks or months later as reduced nesting success, smaller broods, or failed fledging.
Large silk moths make this vulnerability especially clear. Species such as the Polyphemus moth have short adult lives, do not feed as adults, and rely on darkness and precise timing to reproduce. For such species, artificial light is not merely a nuisance; it can be a population-level threat. A single disrupted breeding window can mean zero offspring.
When these moths decline, the loss reverberates outward, affecting predators, plants, and birds that depend on them.
A Biological Signal
Underlying all of these effects is a shared physiological mechanism that has received surprisingly little attention outside of specialist circles: melatonin.
Long studied in humans as a hormone associated with sleep, melatonin is now understood to play a much broader role across taxa. Decades of work by Russell J. Reiter and others demonstrated that melatonin acts as a master regulator of circadian rhythms, seasonal timing, immune function, and reproductive physiology in animals ranging from insects to birds to mammals.
Crucially, melatonin production is strongly suppressed by short-wavelength light. White and blue-rich LEDs are particularly effective at shutting down melatonin synthesis, even at relatively low intensities. Red light, by contrast, has a much weaker effect.
This difference explains why red lighting is used in settings where preserving night vision and circadian integrity matters, such as observatories, submarines, and increasingly wildlife-sensitive environments.
When artificial lighting suppresses melatonin, the consequences extend far beyond sleep disruption. In birds, melatonin helps regulate photoperiodic responses that determine when breeding begins, when molt occurs, and how migration is timed. In insects, melatonin influences daily and seasonal rhythms that govern feeding and reproduction.
In mammals, including humans, melatonin suppression is associated with increased oxidative stress, impaired immune function, and long-term health risks. The same biological signal—light at night—is altering internal chemistry across species, whether or not we choose to notice.
Seen in this light, the experiment in Denmark is not really about bats, moths, or birds alone. It is about acknowledging that urban infrastructure sends powerful biological signals, and that those signals can be redesigned. The choice is not between safety and ecology, nor between progress and nostalgia. It is between treating environmental harm as an unfortunate externality, or recognizing that harm often arises from design decisions that can be revisited.
What makes lighting a particularly instructive case is that the solutions are neither exotic nor radical. Reducing brightness, narrowing spectral output, shielding fixtures, and placing light only where it is genuinely needed are all well within current technological capabilities. These changes do not require moral appeals or perfect behavior from individuals. They work quietly, continuously, and system-wide.
Artificial light at night illustrates a broader truth about modern environmental problems. Many of the most damaging effects are not the result of malice or ignorance, but of inherited assumptions embedded in infrastructure. We light cities the way we do because that is how they have been lit for decades, not because it is biologically benign. The costs—disrupted ecosystems, altered behavior, physiological stress—are paid by organisms that have no voice in planning meetings and little capacity to adapt.
The question raised by Denmark's red streetlights is therefore not whether this approach is universally applicable or whether every city should immediately follow suit. The deeper question is whether we are willing to treat the night itself as something worth designing carefully, rather than as an empty backdrop to be flooded with light. In answering that question, we are also deciding how seriously we take the idea that shared systems should do less harm by default, instead of asking the most vulnerable—human and non-human alike—to simply endure the consequences.