After all, what harm could light do? It’s just light.

The answer is: a lot. The damage of light pollution has only begun to be understand in the last two decades, writes Verlyn Klinkenborg for National Geographic. And that’s not just because the unpolluted night sky is full of a vast world of celestial lights that has struck humans with awe since the beginning. “Ill-designed lighting washes out the darkness of night and radically alters the light levels—and light rhythms—to which many forms of life, including ourselves, have adapted,” she writes. “Wherever human light spills into the natural world, some aspect of life—migration, reproduction, feeding—is affected.”

Sea turtles can’t figure out where to lay eggs, and hatchlings find the bright roadway instead of the sea. Fireflies can’t mate. Migrating birds get confused and fly into brightly lit buildings. In humans, light pollution is associated with depression, sleeplessness and cancer. The darkness of the night is essential for humans and other species, Klinkenborg writes: “We’ve lit up the night as if it were an unoccupied country, when nothing could be further from the truth.”

As humans began to seriously consider the consequences of their light use, countries began to legislate against it. On this day in 2002, the Czech Republic struck back by putting a new law into effect to combat light pollution with a simple (and effective) solution: “From 1 June, all outdoor light fixtures must be shielded to ensure light goes only in the direction intended, and not above the horizontal,” wrote Tom Clarke for Nature in 2002. It was the first national law of its kind in the world.

Outdoor lights now have to be shielded to keep light from spilling out above a certain height, and flat glass rather than curved has to be used, writes Kate Connolly for The Guardian.

The Czech law is still in effect, and other countries and regions have adopted similar measures to the “Protection of the Atmosphere Act.” But a 2016 study found that one third of humanity still can’t see the Milky Way, and in Europe and the United States, more than 99 percent of people live in light-polluted conditions. Anti-light pollution advocates such as the International Dark-Sky Association say there is more to do.

Founded in 1988, IDA is an U.S.-based education and advocacy group advocating against light pollution. It consults on initiatives like the one in Florida to reduce infant sea turtle deaths, and also certifies places that have worked to reduce light emissions, such as—recently—Cedar Breaks National Monument in Utah. The organization has been on the front lines of the fight for dark-sky legislation.

“Electricity is a modern necessity of life,” Franklin Delano Roosevelt once said. It’s true that artificial light has done many positive things for humanity, but like anything else, it has consequences.