— Health: Too much light at night disrupts the body's natural 24-hour day-night cycle and prevents the release of melatonin, a hormone that controls a person's sleep and wake cycles.
— Decreased security: Bright lights produce blinding glares directly into a person's eyes and create black shadows, making it difficult to see the ground and one's surroundings.
— Neighbourliness: Glare from bright lights shining into a yard or window is un-neighbourly. "Full-cutoff" fixtures direct light downward and keep unwanted light within property boundaries.
— Seniors: Lights from houses and streets make it tougher for the elderly to see when they go out at night because the transparency of an older eye is not as good as a younger eye.
— Habitats: Light shining toward wetlands or any vegetated area turns night into twilight, disrupting the lives of their inhabitants — insects, amphibians and other species. Feeding, mating and egg-laying suffer.