Notice more people wearing glasses lately? It's not just a hipster trend—more people then ever actually need them to see long distances.

An article in the scientific journal Nature shows that nearsightedness is reaching epidemic status. And scientists are closer than ever to figuring out why.

Up to 90% of teenagers who live in China are nearsighted, and sixty years ago, only 10 to 20 percent of Chinese people could say the same. And it's not just an Asian phenomenon; more than 2.5 billion people around the world will have myopia by 2020. Experts say the phenomenon could lead to more cataracts, glaucoma, and even blindness down the road.

The rise of myopia (being short-sighted) in Asia over the years is staggering http://t.co/1WkwpIOpBt pic.twitter.com/CYYbn4RgCQ — Cooper Smith (@CooperASmith) March 19, 2015

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The article points to an interesting hypothesis behind the cause of all this nearsightedness: Kids are staying indoors for too long these days. In multiple studies, kids who spent more time indoors had a higher risk of developing myopia. And it doesn't have to do with physical activity, it just has to do with physically being outside and being exposed to bright light. When you're not exposed to outdoor light very much, it inhibits how your eyes grow.

One researcher recommends kids spend at least three hours a day exposed to bright light—the equivalent of sitting under a tree and wearing sunglasses. That's way more than today's kids usually get. If you're already nearsighted, then it's too late for you to fix it with some sunlight – but a nice stroll outside would probably do you some good anyway.