Children who spend more time playing outside may be protecting themselves from needing glasses, according to a study from the Center for Eye Research Australia.

Researcher Mingguang He found that Chinese primary school children who spent an additional 40 minutes a day playing outdoors for three years reduced their risk of myopia (short-sightedness) 23 percent when compared with children who didn't spend extra time outdoors.

"This reduction is clinically important because we targeted young children around 6 years of age, and these children are at greater risk of progressing to severe myopia if they develop myopia early," said He.

"If we can delay the onset of myopia through a simple and cost-effective intervention like this, we can provide great long-term eye health benefits," He said.

A high percentage of young adults in China and other Asian countries have myopia. The Chinese government's Xinhua News Agency reported that 86 percent of high school seniors in Shanghai have the condition.

Previous studies have linked myopia to education, higher incomes, and urbanization, with perhaps long hours spent studying and staring at computers as causes. But a 2008 study found a marked difference in eyesight between young ethnic Chinese students in Singapore and Sydney, suggesting that sunlight could be a key.

Only 3 percent of the Sydney youngsters had myopia compared with 29 percent of those in Singapore.

No one is sure why sunlight appears to protect against myopia, but Ian Morgan, coauthor of the 2008 study told npr.org that he believes sunlight may cause the retina to release dopamine, which inhibits the elongation of the eye that is the main characteristic of myopia.

When He began his study, he and his fellow researchers had hoped their study would show as much as a 50 percent reduction in myopia, and he wonders if even more time spent in the sun would yield improved results.

"School-based initiatives could include even more time for classes outdoors, for example using school recesses to get children outdoors, encouraging parents to bring their children outdoors at weekend, or using new classroom design to provide higher indoor light intensities. All of these strategies will help translate the research findings into community benefit," He said.

The study was published in the Journal of the American Medical Association.