Humans are diurnal creatures, with eyes adapted to living in the sun's light. Because of this, we’ve engineered the night for our own comfort by filling it with light.
Light pollution is largely the result of poor lighting design, which allows artificial light to shine outward and upward into the sky. Wherever human light spills into the natural world, some aspect of life -- migration, reproduction, feeding -- is affected.
Scientists speak of songbirds and seabirds being "captured" by searchlights, circling and circling in the thousands until they drop. Migrating birds are apt to collide with brightly lit tall buildings. Many nocturnal mammals forage more cautiously under the permanent day of light pollution, because they've become easier targets for predators.
Like most other creatures, humans need darkness. The regular oscillation of waking and sleep is a biological expression of the regular oscillation of light on Earth. At least one new study has suggested a correlation between higher rates of breast cancer in women and the nighttime brightness of their neighborhoods.