Unnatural outdoor light at night both wastes energy and is unhealthy for humans and the environment. It can wreak havoc with the circadian rhythms of people and animals, seriously impacting health and disrupting the ecosystem.
Light pollution has a significant impact on wildlife. Nighttime light can disrupt the migration of many birds. Most songbirds migrate at night. Excessive nighttime light can disorient them, altering their path and even causing an increase in building collisions.
Light pollution also has a serious aesthetic impact. There was a day when you could look at the night sky and see it lit up with stars with the bright streak of the Milky Way across the sky. Now only the brightest stars are barely visible. There was a day when kids could go down to the local school yard at night and collect a jar of fireflies in a few minutes. Now, with the impact of light pollution and pesticide use, a firefly is a rare sighting.
A few simple choices can greatly lessen the impact of outdoor lighting.