It’s hard to argue with the idea that getting kids outside is important for their health. The American Academy of Pediatrics encourages doctors to “write a prescription for play” and says that outdoor play in particular is important for “motor, cognitive, social, and linguistic” skills, as well as exercise. But another important benefit of outdoor play is mostly ignored: what it can do for kids’ eyesight.

Nearsightedness has become more common over the past few decades, both in the United States and elsewhere, and scientists aren’t sure why. But they do know that sending your kid outside can help prevent it.

An eye that’s nearsighted, or myopic, doesn’t grow proportionally. It’s usually elongated, making the eye shaped more like a grape than a marble. This means light no longer lands precisely on the retina, which sits at the back of the eye like film in a camera, but instead focuses in front of the retina. A person with eyes that are stretched like this can still see fine up close, but objects farther away look blurry.

It’s not clear what makes some people’s eyeballs grow off-kilter. Genetics are a factor — a child is more likely to become nearsighted if they have one myopic parent, and even likelier with two. But genetics alone can’t explain the sudden surge of nearsighted people. In the early 1970s, myopia was estimated to affect about 25% of Americans between ages 12 and 54. By the early 2000s, that number had jumped to almost 42%. “There’s something that’s happening,” says optometrist David Berntsen, PhD, an associate professor at the University of Houston College of Optometry.

Researchers are actively studying and debating what factors are at work. But one clear, consistent message has emerged from the data: Kids who spend more time outdoors are less likely to become nearsighted.

For example, a 2007 study led by Lisa Jones-Jordan, a research professor at the Ohio State University College of Optometry, followed hundreds of kids in the United States between first and eighth grade. It found that kids who became myopic had spent less time in outdoor activities and sports than kids whose eyesight stayed good. An even larger 2015 study led by University of Melbourne professor Ming He followed first-graders in China, half of whom had 40 minutes of outdoor time added to their school days. After three years, the kids getting extra recess were substantially less likely to have developed myopia. Other studies have had similar results.

The research is especially urgent in East Asia, where nearsightedness in some groups of young people has soared to 80% or higher. The reasons are unclear, though in general, researchers think “high educational intensity” is a factor, He says. The Chinese government is now encouraging more outdoor time for kids as part of a plan to reduce nearsightedness.

Another part of that plan is cutting back on written assignments for very young kids in China. The idea that reading and writing contribute to nearsightedness is common; Berntsen says people have hypothesized for decades that “near work,” or close-up visual activities (watching TV also falls into this category), can make the eye grow longer. But although some studies have found a link between near work and nearsightedness, others have found no connection. Or they’ve found that near work increases after kids become myopic, because their habits change along with their vision. (This might make sense to anyone who, as a child, found reading books more enjoyable than running around a field and hoping a soccer ball wouldn’t smash their glasses.)

Evidence is also mixed about whether outdoor time can help kids who are already nearsighted. “Once the kids are myopic, time outdoors doesn’t seem to affect how fast they progress,” Jordan says. But the question isn’t totally settled, Berntsen says.

How exactly being outdoors protects kids’ eyes is a bit of a mystery. The effect seems to be independent of how much time kids spend on near work — it’s not just that more time outside equals less time reading. But the answer might be as simple as the amount of light kids get, Berntsen says. Or it might have to do with having distant objects in their field of vision, instead of being indoors, where everything is at close range. Another hypothesis involves dopamine: Bright light causes the release of dopamine in the eye, which may tell the eyeball to stop growing.

Myopia usually develops during elementary school. If your kid does end up needing glasses, it likely won’t be a big deal — they’ll have plenty of nearsighted peers. But severe or “high” myopia, which is also becoming more common, can put people at risk for other eye conditions like glaucoma and retinal detachment. So it’s worth trying to bolster kids’ eyesight by giving them plenty of outdoor time. Even if they’re the type who would sooner run away from a soccer ball than toward it, just going for a walk or having their snack in the sunshine is beneficial.

That doesn’t mean bookworms need to read less, especially because reading has its own powerful cognitive and social benefits. Since the connection to near work is hazy, Berntsen says, “The message in general is don’t take the book away from the child, but have them go outside [in addition].”

“Time outdoors is a positive thing, whether you’re nearsighted or not,” he adds. “As long as you’re wearing your sunscreen like you should.”