A person on a lookout above a brightly lit city at night.
Light Initiative

What is light pollution?

Light pollution is light in the wrong place, at the wrong time, in the wrong amount. It is the easiest pollution on Earth to stop.

Artificial light is one of the great inventions of humanity. It lets us work, travel and gather after dark, and it makes streets feel safer. But most outdoor lighting is designed badly: it throws light where nobody needs it. The result is light pollution, and in one lifetime it has erased the night sky for most of the planet’s people.

Four ways light goes wrong

  • Skyglow. The orange or white dome over towns and cities, caused by light scattering in the atmosphere. It can be seen from hundreds of kilometres away.
  • Glare. Excessive brightness that causes visual discomfort and, paradoxically, reduces visibility, especially for older eyes and for drivers.
  • Light trespass. Light falling where it is not wanted or needed, such as into a bedroom window or a nature reserve.
  • Clutter. Bright, confusing groups of light sources, common in over-lit commercial areas.

Why it matters

Astronomy and culture. The night sky is humanity’s oldest shared heritage. Observatories now have to be built on remote mountaintops, and one in three people can no longer see the Milky Way from home.

Wildlife. Life evolved with a day and a night. Many migrating birds fly at night and navigate by the stars; lit buildings attract and disorient them, and building collisions kill hundreds of millions of birds each year in the United States alone. Sea-turtle hatchlings head for the brightest horizon to find the sea; lights inland lead them the wrong way. Insects swarm around lamps until they die, contributing to insect decline. Bats, frogs and plants all change their behaviour under artificial light.

Loggerhead sea turtle hatchlings crossing a sandy beach towards the waves at dawn.
Loggerhead sea turtle hatchlings heading for the sea at Cape Hatteras National Seashore, North Carolina. On a naturally dark beach the brightest horizon is the sea, which is why lights behind the dunes can lead them astray. Photo: Cape Hatteras National Seashore, US National Park Service, public domain

Human health. Light at night, especially blue-rich light, suppresses melatonin, the hormone that regulates sleep, and shifts the body clock. Research links chronic disruption of the body clock to poorer sleep and other health effects; the evidence on specific diseases is still developing, and we follow it without drawing conclusions it does not support.

Energy and money. Light that shines upward lights nothing. Every unshielded lamp wastes part of the electricity it uses, and the emissions that come with it.

Safety. More light does not automatically mean more safety. Glare creates shadows where the eye cannot see, and studies of street lighting and crime give mixed results.

80%of people live under light-polluted skies (Falchi et al., 2016)
9.6%yearly brightening of the sky seen by citizen scientists (Kyba et al., 2023)
365 to 988 millionbirds killed by building collisions a year in the US (Loss et al., 2014)

The night is half of the life of this planet. We have been lighting it as if nobody lived there.

The Nile delta and Cairo at night from the International Space Station.
Cities are visible from orbit mostly because of light that escapes upwards and is wasted. Credit: NASA

The fix is simple

Most environmental problems are complex and slow. This one is not: the solutions are cheap, available today, and work the instant they are installed. Read how to light outdoors wisely.

Proven solutions

Light pollution is one of the few kinds of pollution that disappears the moment its source is fixed, and real towns, countries and buildings have already shown how much can be won back.

  • Flagstaff, Arizona: a city that kept its stars. Flagstaff’s night-sky protection goes back to 1958, when the city began regulating outdoor light, and in 2001 it became the world’s first International Dark Sky City. Its rules rest on three things: amber light for most outdoor areas, full shielding, and a cap on the total amount of light on business and other non-residential land. Calibrated measurements made in 2016 and presented to the American Astronomical Society in 2017 found that the light dome over Flagstaff, a city of about 80,000 people, was about 90 percent fainter than the one over Cheyenne, Wyoming, a city of similar size (Flagstaff Dark Skies Coalition).
  • Tucson, Arizona: new street lights, dimmed at midnight. In 2016 and 2017 Tucson replaced its street lights with fully shielded, warm white LEDs of 3000 K, and more than 19,000 of them can be dimmed from a central system. Most are turned down to 60 percent power at midnight every night. In a 2019 test that dimmed them further, to 30 percent, the sky overhead in the city centre grew about 5 percent darker as soon as the lights dimmed. The same study estimated that the city’s street lights produce only about 14 percent of the light behind Tucson’s skyglow; the rest comes from other sources, such as lights run by neighbouring towns and private owners, signs, car parks and buildings (Barentine et al., 2020). The lesson: street lights are a good start, but the sky only fully returns when every kind of outdoor light follows good rules.
  • France: a national switch-off. Since a government order of December 2018, shop windows in France must be dark by 1 am, or one hour after closing if later, and car-park lighting must go off two hours after the car park closes. New lights must send less than 1 percent of their light upwards and, in most places, be no colder than 3000 K, with 2400 K in nature reserves (Arrêté du 27 décembre 2018). More than 1,000 French towns and villages now carry a label for good night-time lighting (see the organisations below).
  • Whole landscapes kept dark. In 2011 the Channel Island of Sark became the world’s first Dark Sky Island, showing that a whole inhabited community can light for people and still keep its stars (Sark). The Pic du Midi in the French Pyrenees, home to a historic observatory, became an International Dark Sky Reserve in 2013 (Pic du Midi).
Two park rangers in silhouette beside a large telescope against an orange evening sky.
Rangers set up telescopes for a star party celebrating their park's certification as an International Dark Sky Park in the United States. Photo: Chris Wonderly, US National Park Service, public domain
  • Chicago: fewer lit windows, far fewer dead birds. Two decades of daily counts at one large lakeside building in Chicago showed that halving the area of lit windows cut bird collisions about 11-fold in spring and 6-fold in autumn. Keeping lights at the lowest levels ever recorded there could prevent about 60 percent of the deaths (Van Doren et al., 2021).

Organisations doing this work

These non-profit groups have measurable results and publish what they do. If this page has moved you, please look at their work and consider giving to them directly, through their own websites. The Local Solar System Foundation has no affiliation with any of them and receives nothing from them. Before you give, read each organisation’s own annual reports and accounts.

  • DarkSky International (United States, a tax-exempt non-profit since 1988). Founded in Tucson, it runs the International Dark Sky Places programme that certified Flagstaff, Sark and the Pic du Midi, and the DarkSky Approved scheme for fixtures that meet its lighting standards. It also works with cities and councils on lighting codes.
  • Flagstaff Dark Skies Coalition (United States, tax-exempt non-profit). Founded in 2000, it proposed the Dark Sky Places programme in 2001 and now documents “the Flagstaff Solution” so that other towns can copy it.
  • ANPCEN (France, Association nationale pour la protection du ciel et de l’environnement nocturnes). It campaigned for France’s lighting rules and runs the “Villes et Villages étoilés” label: by September 2025, 1,062 towns and villages held it for turning down, shielding or switching off their lights.
  • CPRE, the countryside charity (England, registered charity 1089685). It runs the annual Star Count, in which thousands of people count stars from home, and publishes maps of light pollution and dark skies across England to help councils protect them.
The glow of a city's lights on the horizon behind the domes of Kitt Peak National Observatory under a starry sky.
The lights of Tucson, Arizona, glow on the horizon beyond the telescopes of Kitt Peak National Observatory. Tucson's shielded, dimmable street lights help, but as the 2019 test showed, every kind of outdoor light has a part to play. Photo: KPNO/NOIRLab/NSF/AURA/P. Marenfeld, CC BY 4.0

How you can help

  • Switch off what you do not need, especially decorative and all-night lights.
  • Shield and aim down every outdoor light you own; choose warm bulbs of 3000 K or lower.
  • Close curtains at night so indoor light stays inside.
  • Talk to neighbours about glaring lights, with a practical fix in hand.
  • Ask your council and local businesses for shielded, warm, dimmed lighting and for signs to go off after closing time.
  • Protect migrating birds: in spring and autumn, turn off unnecessary lights in tall buildings at night.
  • Count stars with Globe at Night and start a dark-sky group through the contribution board.

Sources and further reading