Experts estimate one-third of the world's population will be affected by myopia - nearsightedness - by the end of the decade, and now researchers think they know the cause: Young people are spending too much time indoors.

It's easy to go for the obvious culprit, that they have their nose in a book or in front of a computer screen.

Researchers say it's tempting to believe there's a link between an increase in myopia and a strong emphasis on education, particularly in Asia, where the incidence of myopia has skyrocketed.

It's not coincidence, they say, that the average 15-year-old in Shanghai is inside doing homework for 14 hours a week. East Asia in particular has seen an unprecedented rise in myopia; 60 years ago, 10 percent to 20 percent of the population of China was short-sighted. Today, up to 90 percent of teenagers and young adults have been diagnosed with the vision problem. In Seoul, South Korea, a survey of 19-year-olds found 96.5 percent of them were myopic.

The prescription, however isn't less reading or computer time. Researchers have found the increase in myopia could be reduced by having students get at least three hours of bright sunlight a day.

And book work isn't necessarily the cause - rather it's the result of not spending enough time outdoors in bright light, some researchers suggest.

After studying more than 4,000 children at Sydney primary and secondary schools for three years, researchers from the University of Technology in Sydney found that children who spent less time outside were at greater risk of developing myopia. That went along with results from a study of 500 California eight- and nine-year-olds who started with healthy eyesight but after five years, one in five of the kids had developed myopia, researchers from the Ohio State University College of Optometry in Columbus learned, "and the only environmental factor that was strongly associated with risk was time spent outdoors."

Now much of the world is beginning to see problems with myopia, researchers say; in the United States and Europe the incidence of the condition is double what it was a century ago. Some 2.5 billion people - around third of the global population - could be myopic in another 5 years, experts say.

"We are going down the path of having a myopia epidemic," says Padmaja Sankaridurg, head of the myopia program at the Brien Holden Vision Institute in Sydney, Australia.

In addition to time outside getting kids' eyes away from books and computers, there is growing evidence that brighter light outdoors can help maintain healthy vision. The additional activity can also help fight obesity. 

"It probably also increases physical activity, which decreases likelihood of obesity and enhances mood," says Kathryn Rose, head of orthoptics at the University of Technology in Sydney. "I can only see it as a win - and it's free."

Australian researchers have found kids could keep the vision healthy by spending 3 hours per day in light of 10,000 lux or more, about the level of sunlight on a bright day. A lux measures the amount of light output in a given area, the light intensity.

Inside a home, or even in a well-lit classroom, light levels are usually no more than 500 lux, they say.

Other researchers suggest there needs to be more data to support the idea that more light will reduce the incidence of myopia. Most studies rely on data estimates of time spent outdoors, and sometimes estimates don't match reality, researchers note.

Ian Flitcroft, a myopia specialist at Children's University Hospital in Dublin, isn't sure light is the key protective factor when kids play outdoors. He says that the greater viewing distances outside could affect myopia progression, too. "Light is not the only factor, and making it the explanation is a gross over-simplification of a complex process," he says.

Scientists say they're not completely sure what mechanism is involved in bright light preventing myopia, but one theory holds that light stimulates the release of dopamine in the retina, with neurotransmitters blocking changes in shape in the eye during development that can lead to nearsightedness.

The study was published in the journal Nature.