This article posted at Telegraph.co.uk
Researchers for the Australian government found that spending two or three hours outside each day halves the chance of developing the condition.
They believe that sunlight triggers the release of chemicals preventing the distortion of the eyeball which causes the condition, also known as Myopia.
The findings will question the belief that short-sightedness is brought on by excessive TV watching, computer use or reading in dim light.
The Australian researchers compared the vision of six and seven-year-olds in Singapore and Australia. Thirty per cent of the Singaporean children were short-sighted - ten times the Australian rate.
A similar pattern emerged when the analysis was limited to children of Chinese descent, meaning the difference could not be explained by genetics.
Both groups spent a similar amount of time reading, watching television and playing computer games. But the Australian children spent an average of two hours a day outdoors - 90 minutes more than their Asian counterparts.
Professor Ian Morgan, of the Australian Research Council's Vision Centre, said: "We're seeing large increases in myopia among children in urban societies all around the world - and the outstanding common factor may be less and less time spent outdoors.
"The idea that reading makes you short-sighted has been popular for a couple of hundred years. But recent data shows that the time spent indoors is a more important factor."
Professor Morgan believes natural light - which can be hundreds of times brighter than indoor light - triggers the release of dopamine, a chemical which stops the eyeball from growing out of shape and causing myopia.
Professor Morgan believes that humans were naturally long-sighted but there was a dramatic rise in myopia once people began intensive schooling and spent little or no time outdoors.
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