Light Pollution: Light pollution is excess or misdirected artificial light at night that disrupts natural darkness. It’s growing ∼9-10% per year globally., Types: Skyglow (bright domes over cities), glare (blinding brightness), light trespass (unwanted light spill), and clutter (confusing over-lit areas)., Cause: Bad lighting design — unshielded fixtures, over-illumination, 24/7 use, and cool-white LEDs with high blue content., Human health: Suppresses melatonin, disrupts sleep, and is linked to cancer, obesity, diabetes, depression, and insomnia. WHO lists circadian disruption as a probable carcinogen., Ecological harm: Kills insects drawn to lights, disorients migrating birds causing ∼1 billion U.S. building collisions yearly, leads sea turtle hatchlings inland to die, and alters plant cycles., Other impacts: Blocks star visibility for 80% of North Americans, wastes ∼35% of outdoor lighting energy — $3.3B/year in the U.S. Glare can also reduce safety, not improve it., Key point: It’s reversible. Turning off lights or using shielded, warm, motion-sensor fixtures fixes it instantly and saves money. Cities like Flagstaff cut skyglow >60% with ordinances.