Americans are losing their vision. Literally.

In the past 30 years the prevalence of myopia in the U.S. has increased 66 percent (from 25% of Americans aged 12-54 in the early 1970s, to over 40% of Americans today, according to researches at the NIH’s National Eye Institute). Genetics are known to be a factor, but that’s a dramatic increase, so researchers figure something else has changed.

It turns out your parents were wrong about why you need glasses–at least in the case of near-sightedness. For many years we all heard the same advice: don’t read in dim light, don’t use a flashlight to read under the covers, don’t watch too much TV.

Researchers are learning that the real reason for the dramatic surge in myopia is that we are becoming a nation of dedicated indoorsmen.

Don Mutti, of the College of Optometry at Ohio State University, says “If you have two nearsighted parents and you engage in a low level of outdoor activity, your chances of becoming myopic by the eighth grade are about 60 percent,” according to a story reported Monday on NPR. “If children engaged in over 14 hours per week of outdoor activity, their chances of becoming nearsighted were now only about 20 percent. So it was quite a dramatic reduction in the risk of becoming myopic.”

Being a bookworm turns out not to be a problem at all, unless it keeps you from being outdoors. In the case of watching TV or playing too many video games, it’s not staring at the screens that causes near-sightedness, it that addiction to screens displaces outdoor play. And it’s not that exercise helps avoid mypopia–kids who got plenty of indoor exercise still have high rates of near-sightedness (although a high level of physical activity outdoors and indoors is great for kids for lots of other reasons–think of the obesity epidemic, and the rise in diabetes). Just being outside reduces the risk of developing myopia and needing glasses.

I asked Mutti how much time outdoors is the right amount. He said, “What we found was that 14 hours outdoors per week was the lowest risk group. However, this is not the same thing as knowing what the effect of sending kids out for 14 hours per week really is. That takes a randomized clinical trial — and that has not been done yet. The good news is that exercise is good regardless of whether it prevents myopia.”

For now, scientists are investigating the role of outdoor light on retinal development, thinking that outdoor light affects children’s developing eyes differently. According to Mutti, it could also be the case that opportunities to focus on far, clearly-lit objects plays a factor.

Mutti noted that the dramatic rise in myopia is likely not a result of better screening. The data in the US come from a standardized national screening effort–the National and Nutrition Examination Survey, unique because it combines interviews and physical exams–that has been uniform across the years.

As reporter Joe Shapiro of NPR noted another reason all this matters: “Eye care is expensive.” Lead researcher on the rise in myopia, Dr. Susan Vitale of the National Eye Institute, made a back-of-the-envelope estimate that the increased costs of glasses, contacts, and eye exams has taken the costs of treating myopia from about $2 billion a year in the 1970s, to over $3 billion today.

The statistic bears repeating:
Two hours a day outside reduces the chance of becoming nearsighted to one-in-five, even with two near-sighted parents. If those kids stay inside much of the time, that risk goes up to 60%.

Whether it’s watching birds or baseballs, kids that spend a lot of time outside have better vision.

Related articles
Videophilia replacing love of nature
Family Fun activities at Flourish Online (including tips for getting kids outdoors)