Kelsey Hambor

We tend to think of light as a good thing. It helps us see where we’re going, makes streets and buildings easier to navigate, and allows us to work, travel, garden, exercise, and spend time outside after dark. As winter approaches, those extra hours of artificial light can feel especially useful in Minnesota. But what happens when we forget that nighttime belongs to animals, too?

For billions of years, life revolved around a predictable cycle of light and darkness. The sun went down. The stars and moon provided the available light. Animals adapted to those rhythms, and many still depend on them.

Nearly half of the species on Earth are nocturnal. For these animals, darkness isn’t simply the time when humans aren’t around. It is part of their habitat. Natural darkness helps animals navigate, find food, avoid predators, nest, mate, and regulate behaviors tied to the day-night cycle.

Artificial light changes that habitat. Light pollution can interfere with feeding, reproduction, migration, navigation, sleep, and predator-prey relationships. Researchers have documented effects across a remarkable range of animals, including birds, mammals, amphibians, reptiles, fish, insects, and other invertebrates. Plants can be affected, too.

For example, migrating birds can be attracted and disoriented by artificial light, drawing them toward cities and illuminated buildings. Birds that migrate at night use natural light cues, including the stars and moon, to navigate. Artificial lighting can pull them off course, cause them to circle illuminated structures and waste valuable energy, and increase their risk of collisions. The U.S. Fish & Wildlife Service estimates that nearly one billion birds die each year in the United States from collisions with buildings and other structures.

Insects, including moths, lightning bugs, and other nocturnal pollinators, are also affected. Many have evolved to navigate and forage under very low levels of natural light. Artificial lighting can draw them away from flowers, interrupt feeding and mating, and interfere with pollination.

Plants respond to changes in light too. Artificial light at night can interfere with natural seasonal and daily signals that regulate growth and other biological processes. Changes in nighttime lighting can also affect the insects and other animals that plants depend on, creating effects that extend beyond a single species.

None of this means we need to turn off every light and sit in the dark. It means we should be more intentional about the light we use. We should ask ourselves: do we need this light? Could it be turned off when we’re not using it? Could it point downward instead of into the sky? Could a motion sensor replace a light that’s on all night? Could we use less intense or warmer-colored lighting?

These are some of the principles behind the Dark Sky movement: use light only when it is needed, where it is needed, and at the lowest useful level. Shielding lights, reducing brightness, using timers or motion sensors, and choosing warmer-colored light can all help reduce unnecessary impacts on wildlife. 

We have changed the nighttime environment in ways that are convenient for us, but not necessarily for the animals and plants sharing that environment. The night is a biological system, not simply an empty space between one day and the next. Sometimes protecting wildlife begins with something as simple as turning off the light.