Ecological light pollution refers to how artificial light affects individual living things and the overall balance of ecosystems.

Artificial light can have different effects on organisms. For example, it can help predators see prey more easily, but it can also cause harm or death, such as when moths are drawn to light sources and die from the heat. Sometimes, light at night can be both helpful and harmful to a species. Humans benefit from using artificial light indoors because it allows more time for work and activities, but this light can also disrupt the body's natural sleep-wake cycle, leading to health problems.

The way artificial light affects individual species can change the ecology of an area. If two species share the same role in an ecosystem, artificial light might change how common each species is if they react differently to nighttime light. For example, some spiders avoid areas with light, while others build webs near light sources. Since lamp posts attract insects, spiders that tolerate light may have more food and become more common than those that avoid light. These changes can cause other effects in the ecosystem, such as altering food webs. For instance, if insects that are active at night change their behavior, it could affect plants that bloom at night, which in turn might impact daytime animals that rely on these plants for food or shelter.

Adding artificial light at night is one of the biggest changes humans have made to Earth. This change is as significant as other major environmental issues, such as pollution, changes in land use, and climate change caused by rising levels of greenhouse gases.

Natural light cycles

The use of artificial light changes natural light patterns that are created by the movement of Earth, the Moon, and the Sun, as well as by weather patterns.

One clear change caused by artificial light at night is the loss of darkness. The day/night cycle, also called the diurnal cycle, is a strong signal that influences the behavior of most animals. Many animals are either active during the day (diurnal) or at night (nocturnal). If a nocturnal animal needs very dark conditions to be active, it may struggle to survive in areas with light. The strongest effects of artificial light are near streetlights and buildings, but the soft light from skyglow can spread far from cities, sometimes hundreds of kilometers away. Even blind individuals are often in sync with the day/night cycle, with about one-quarter of fully blind people following it.

Earth’s tilt causes seasons outside the tropics. The amount of daylight, or photoperiod, is a key signal for seasonal behaviors, such as mating, in non-tropical animals and plants. Artificial light at night can cause "seasons out of time," which may change behaviors, body temperature regulation, and hormone levels in affected organisms. This can lead to a mismatch between an organism’s body functions and the actual season, possibly disrupting reproduction, hibernation, and migration.

Some animals, such as coyotes, bats, toads, and insects, depend on the moon’s cycle for their behavior. In cities, skyglow often makes the night brighter than a full moon, which can change these behaviors and reduce the animals’ ability to survive. Scientists have also found that artificial light at night can affect the daily biological clocks, or circadian rhythms, of nocturnal animals.

In untouched areas, clouds block out stars and make the night as dark as possible. In cities and suburbs, clouds make skyglow more noticeable, especially for longer wavelengths of light. This means that areas near cities are much brighter at night, and truly dark nights do not happen in these places.

Because of this, the night sky is becoming harder to see in some areas, especially in large cities. Studies show that the night sky is becoming 10% less visible worldwide each year.

Effects on organisms

Terrestrial species are those that live mostly or entirely on land. This includes animals such as birds, insects, and land mammals.

Insects are often drawn to artificial light, which is one of the most well-known examples of how light at night affects living things. When insects are attracted to lights, they may be harmed by exhaustion or by touching the light source. They also become easier targets for predators like bats.

Insects respond differently to various types of light. Many species can see light that humans cannot, such as ultraviolet and infrared. Nocturnal insects, like moths, use light to help them fly at the correct height by facing the light with the top of their body. However, near artificial lights, this behavior can cause them to circle the light repeatedly, trapping them. Because moths are sensitive to ultraviolet light, they are more attracted to white or blue lights than to the yellow light from low-pressure sodium lamps.

Dragonflies see horizontally polarized light as a sign of water. Because of this, they may mistake dark, flat, reflective surfaces like asphalt roads or car tops for water. Dragonflies searching for water to drink or lay eggs may land on these surfaces and die from dehydration or overheating.

Light pollution can harm fireflies, which use their own light to find mates. This can lead to fewer fireflies. Fireflies are easy for people to spot and respond quickly to changes in their environment, making them useful for studying the effects of artificial light at night.

Artificial light at night may contribute to the decline of amphibian populations. Many amphibians are active at night, and changes in their natural light-dark cycle can disrupt their biological rhythms. This can affect their growth, how long it takes them to change from larvae to adults, and their feeding, mating, and survival behaviors.

Most bats are active at night and are greatly affected by artificial light. These effects vary depending on the species and the situation. Artificial light can interfere with their migration, travel to hunting areas, finding shelter, and foraging for food.

Birds often migrate at night because it is cooler, which helps them avoid dehydration. However, city lights can make it harder for them to navigate using the stars. Lights on tall buildings can confuse migrating birds, leading to collisions. It is estimated that between 365 and 988 million birds collide with buildings in North America each year. Turning off lights during migration periods can help reduce these deaths. Programs like the Fatal Light Awareness Program (FLAP) work with building owners in cities like Toronto to lower bird deaths.

Birds near offshore oil and gas platforms can also become disoriented by lights. Studies by companies like NAM and Shell led to the use of new lighting technologies in the North Sea. One experiment on a Shell platform reduced the number of birds circling the structure by 50–90%. Juvenile seabirds may also be confused by lights when leaving their nests, leading to high death rates. Rescue programs on islands help save many seabirds.

Ceilometers, which are powerful lights used for measuring weather, can trap birds in their beams. Birds may become exhausted or collide with each other. In 1954, 50,000 birds from 53 species were killed at Warner Robins Air Force Base due to ceilometers.

Light pollution can cause birds to reach sexual maturity earlier and forage more aggressively than usual.

A study in Science used data on 583 bird species to show that light pollution increases the time birds spend vocalizing by about 50 minutes. This effect is strongest in species with large eyes, open nests, migratory habits, and large ranges, especially during the breeding season.

Lights from coastal developments can prevent sea turtle mothers from finding their nesting areas. Turtle hatchlings rely on moonlight to find the ocean but are often drawn to artificial lights on land, leading to their deaths.

Artificial light harms trees and plants, especially in autumn. Plants use the length of daylight to know when seasons are changing. When daylight decreases, plants prepare for winter by shedding leaves. Artificial light can trick plants into thinking it is still summer, causing them to keep their leaves longer. This wastes energy that should be saved for winter. Light pollution can also keep plant pores open at night, making them more likely to get sick.

In spring, artificial light can cause plants to start growing leaves too early, before the weather is warm enough. This can harm small plants that lack enough protection from the cold. Light pollution may also cause trees to begin growing new leaves earlier than normal.

Aquatic animals live in water, including fish, dolphins, and turtles. Artificial light can harm marine ecosystems. As of 2010, about 22% of the world’s coastline was affected by artificial light at night.

Zooplankton, like Daphnia, move up and down in lakes during the day. In lakes with fish, they avoid light because fish hunt them. Artificial light from skyglow limits how high they can go at night. Zooplankton eat algae, so their reduced activity may lead to more algae growth, which can harm plants and lower water quality.

Effects of different wavelengths

The effect of artificial light on living things depends on the color of the light. Humans cannot see ultraviolet light, but scientists who study insects often use it to attract them. Blue light is more likely to harm mammals because their eyes have special light-sensitive cells that are most responsive to blue light. In humans, exposure to blue light at night can reduce the production of melatonin, a hormone that helps control sleep, which may lead to sleep problems and increased stress.

In nature, a process called Rayleigh scattering makes the night sky appear brighter. This happens because shorter wavelength light, like blue light, spreads more easily in the atmosphere than longer wavelength light, such as red or green. If traditional streetlights are replaced with white LED lights, which emit more light in the blue range, the impact on the environment could be greater, even if the total amount of light produced is lower.

Polarized light pollution

Artificial flat surfaces, such as glass windows or asphalt, reflect highly polarized light. Polarized light pollution happens because of the materials and technologies we use in construction. This type of pollution can be lessened by carefully choosing materials for buildings. An example of polarized light is the reflection seen on the glass of a watch. Many insects are drawn to polarized surfaces because polarization often signals the presence of water. This effect is called polarized light pollution. While it is a type of ecological light pollution, the term "ecological light pollution" typically refers to how artificial light affects living things.

At night, the polarization of moonlight in the sky is greatly reduced when urban light pollution is present. This is because light scattered from cities is not strongly polarized. Scientists believe that many animals use polarized moonlight to navigate. Therefore, the reduction of polarized moonlight is another harmful effect of light pollution on ecosystems.