If you’re a parent struggling to get your kids’ off their devices and outdoors to play, here’s another reason to keep trying: Spending at least two hours outside each day is one of the most important things your kids can do to protect their eyesight.

“We think that outdoor time is the best form of prevention for nearsightedness,” says Dr. Noha Ekdawi, a pediatric ophthalmologist in Wheaton, Ill.

And that’s important, because the number of kids with nearsightedness – or myopia – has been growing rapidly in the U.S., and in many other parts of the world.

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Wu convinced his son’s elementary school to increase outdoor time. He also recruited a control school. A year later, his son’s school had half as many new myopia cases as the other school. “We saw the results – they were very successful,” Wu says.

He did more research, at more schools, and eventually convinced Taiwan’s Ministry of Education to encourage all primary schools to send students out doors for at least 2 hours a day, every day. The program launched in September 2010. And after decades of trending upward, the rate of myopia among Taiwan’s elementary school students began falling – from an all-time high of 50% in 2011 down to 45.1% by 2015. It’s a major achievement, says Ian Morgan.