From fireflies to fish, light pollution is damaging the environment – what can we do about it?

This article has been updated.

Before the invention of the electric light, humans largely lived, slept, and worked in a way governed by the light of day. This natural rhythm of day and night is encoded into all living things, from people to plants.

But with Thomas Edison's 1880 patent for the incandescent lamp paving the way, electric lighting has been standard in most houses in the developed world since the mid-twentieth century. Since then, we've been lighting our homes, offices and streets to the point where a 2016 paper concluded that night-time light pollution covers nearly 80% of the planet. With increasing technological advancements and urbanization, it's continuing to get ever lighter, with the world's artificially lit areas expanding at an estimated 2.2% a year.

And this dramatic shift away from natural light cycles is affecting wildlife, biodiversity, human health and global energy efficiency.

While a broadly accepted definition of light pollution is the light generated by human activity that makes it difficult to see things in the sky at night, a growing body of evidence suggests the impact goes beyond merely blocking the potential for stargazing.

How dark the night sky is affects wildlife, including bats, migratory birds, and insects. Light pollution threatens the health and natural cycles of humans, wastes energy and money, and disrupts ecosystems.

The consequences of light pollution are wide-ranging and can pose serious risks to many aspects of life on Earth. Here are a few of the associations that are proven or under investigation:

1. Light pollution interrupts wildlife patterns and harms biodiversity

Studies show that birds, bats, and insects are among those in the natural world affected by light pollution.

Migratory birds rely on natural light to guide them and interruptions can lead to collisions and incorrect navigation on their long-distance journeys. Sea turtles confuse artificial light for moonlight and are drawn away from the ocean, with millions of hatchlings dying in this way each year.

Artificial light disrupts nocturnal moths and may have reduced the number of caterpillars by half, according to another study. And the mating flashes of fireflies meanwhile can be disrupted by even momentary flashes from vehicle headlights. This has knock-on implications for those who feed on them.

Excess light can also tamper with the habits and reproductive cycles of bats.

The patterns and habits of other animals active at night – including owls, badgers, mice and frogs – are also impacted. For example, animals that use the cover of night to hunt, or to hide, are being affected, disrupting food webs. Bats have been shown to steer clear of lit-up areas because it exposes them to predators such as owls.

The effects aren't limited to land animals – fish also adapt their behaviour in response to light, for example shifting to deeper waters.

2. Light pollution is associated with risks to human health

Humans are not exempt from the effect artificial light has on animals, with studies showing that it disrupts the body’s biological clock, hampering sleeping cycles.

What’s worse, artificial light was found in a study to be “significantly correlated for all forms of cancer” including lung, breast, colorectal and prostate cancers individually.

“Immediate measures should be taken to limit artificial light at night in the main cities around the world and also inside houses,” the authors of another study wrote.

3. Light at night wastes energy and money, contributing to climate change

Conserving energy is one of the easiest ways to combat climate change. Leaving artificial lights on – for example, in offices or stores after hours – is called “over illumination”, and curbing it is one of the easiest ways to cut back on light pollution and save energy.

A European Commission study showed that city lights decrease the nighttime cleansing of the atmosphere, and reduce the ways nitrogen oxides are removed.

The World Economic Forum's report Nature Positive: Leaders’ Insights for the Transition in Cities discusses how we can combat the challenges of urbanization and its effect on the environment. It highlights the role that private institutions and civil society play in ensuring that urban development happens in harmony with nature.

The good news is that light pollution is reversible - and this sets it apart from many other forms of pollution.

So what steps can be taken? The International Dark-Sky Association has the following tips:

1. Reduce the use of light, installing it only where it’s really needed and at the lowest brightness possible.

2. Use controls like dimmers, timers and motion sensors to make the lights as efficient as possible.

3. Use shields to direct light toward the ground, reducing glare.

4. Use warm-coloured lights where possible and limit the use of blue wavelength lights, which are thought to be the most disruptive.

"Unlike many other environmental issues facing the world, solving light pollution is straightforward, saves money, and delivers immediate results," says Ruskin Hartley, Executive Director at International Dark-Sky Association.