OUT you go, dear, into the fresh air. Better still, out to the mountains with you and walk. Breathe. Air like wine. Look -- the rolling hills of Ireland.

Sorry to sound like a scoutmaster, but shocking research has apparently discovered the reason for an increasing proportion of the world's kids growing up into specky four-eyes. Those types who said kids who stayed indoors with their nose in a book were right. Not about the nose in a book -- about being indoors.

There's a worldwide epidemic of short-sightedness (or near-sightedness, as the Americans call it). Myopes are everywhere.

A study by Susan Vitale, of America's National Eye Institute, found that myopia had risen from 25pc of the US population in the early 1970s to 41.6pc in the early 1990s.

By 2020, it's estimated that 2.2 billion people worldwide will be myopic.

It's in your genes -- but not really. Humans are naturally slightly long-sighted -- handy for spotting those gazelles on the ancient plains -- but lifestyle has crippled our eyesight.

Kathryn Rose, of the University of Sydney, carried out a monster study of 2,300 12-year-old Australians, and found the answer: it's light. If you're outdoors, your eyes are unlikely to become crippled by short sight.

Her research was backed up by a study of 1,250 Singapore teenagers by Seang-Mei Saw, of the Yong Loo Lin School of Medicine.

Dr Rose continued her work with an even larger sample. "In 2005, we concluded a study of 4,000 schoolchildren," she told me on a dodgy mobile-phone line from Australia.

"We were looking at eye health in Australian children on a population basis sample, but we were also very interested in the development of myopia.

"Myopia had increased very dramatically over a period of decades -- especially in East Asian countries -- which implied that there were environmental factors."

The obvious culprit was close work -- take your nose out of that computer, you -- but studies hadn't borne out the link.

Some scientists were now suggesting that it was the lack of outdoor exercise that caused the eyes to freeze in place and lose their ability to change focus -- the basic problem with myopia. Dr Rose wanted to test this theory.

"We did a very complicated study, looking at a range of environmental factors," she said. "What we were able to demonstrate was that time spent outside was, in fact, protective against developing myopia.

"Crucially, because we'd asked our questions in such detail, we were also able to show that it wasn't related to activity itself.

"It was actually related to the hours spent outside."

The scientists, fascinated, started doing what scientists do best: hypothesising. Could it be retinal dopamine, they wondered? It has been well known for some time that retinal dopamine is an inhibitor of eye growth and is released in response to light.

"The jury's still out on whether it's retinal dopamine, but the jury's not out on the effect of light," said Dr Rose.

"What I've really enjoyed from this is the fact that now parents are telling their children to go outside and play."