Places with truly dark skies are shrinking. The glow from street lights and buildings is far-reaching. Light doesn’t respect boundaries, and careless use can see it spread for miles from towns, cities, businesses and motorways. There is growing evidence that lighting up the night is affecting biodiversity and the welfare of creatures both nocturnal and diurnal, while also leading to insomnia and other health issues for humans. For regional councils, lighting can be a statutory nuisance in the same way that noise is.
Huge numbers of species use daylight information as a clock. Turtle hatchlings can be disoriented by skyglow and move away from the sea. Insects attracted to light are easier prey, which unbalances ecosystems. The International Dark-Sky Association lists more than 2,600 studies on the impacts of artificial light on clownfish, mice, worms, bats, coral reefs, katydids and tree sparrows.
Birds may decide to migrate or lay eggs at a particular time of year, based on how much light there is. If they arrive too soon, before insect populations have boomed, they risk starvation. Even short periods of light can have an impact. Memorial light beams at the site of the World Trade Center in New York are shone on one day a year, 11 September, but they lure as many as 160,000 migratory birds off course, circling the beams. Now they are switched off periodically to let the birds disperse.