Why does light pollution matter?

Light pollution is excessive or misdirected artificial light. It is unusual among environmental problems in being immediately reversible — switching a light off ends it instantly — yet it continues increasing, by satellite measurements at several percent a year.

The types:

Skyglow — the diffuse brightening of the night sky over populated areas, caused by light scattering off atmospheric particles.

Light trespass — light falling where it is not wanted, such as a streetlight into a bedroom.

Glare — excessive brightness causing visual discomfort, which frequently makes seeing worse rather than better.

Clutter — confusing groupings of bright sources.

Why it matters:

Wildlife. The effects are well documented and substantial. Insects are drawn to lights and die there or are removed from breeding and pollination, and artificial light is one of the contributors implicated in insect decline. Migratory birds navigating by stars become disoriented, and collisions with lit buildings kill very large numbers. Sea turtle hatchlings orient toward the brightest horizon and move inland toward development instead of the sea. Bats, moths, amphibians and nocturnal mammals all show altered behaviour.

Plants respond to day length, and artificial light shifts the timing of leafing and flowering.

Human health. Light at night — particularly blue wavelengths — suppresses melatonin and disrupts circadian rhythm. Associations with sleep disruption are well established; links to metabolic and other health outcomes are an active research area with less certainty.

Loss of the night sky. A substantial majority of the world's population, and the overwhelming majority in Europe and North America, cannot see the Milky Way from where they live. This is a genuine cultural loss.

Wasted energy and emissions from lighting the sky.

What fixes it: shielded fixtures directing light downward, warmer colour temperatures, lower intensity, and timers or motion sensors. LEDs helped and hurt — more efficient, but often bluer and used more liberally.