In most major metropolitan areas, you can count the stars visible in the night sky on your fingers. Now, the phenomenon is spreading; due to urban sprawl, bright artificial lights are drowning out the darkness in more and more of the world. That’s bad news for astronomers, public energy budgets — and many plant and animal species. Since the 1970s, scientists have been studying the effect of light on organisms’ biological clocks and nocturnal behavior patterns. They have found that in urban areas, night migrating birds orbit bright lights until they drop with exhaustion or collide with buildings or other birds; that bright lights discourage female sea turtles from laying eggs; that the nighttime travel habits of mountain lions are disrupted by light pollution; and that small invertebrates that normally rise at night to feed on surface algae in lakes and ponds become less active as light levels increase, possibly leading to more algae blooms and lower water quality. In response, nine U.S. states have adopted “dark sky” provisions and 11 more are considering similar measures.