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The time children spend outdoors could be linked to a reduced risk of being short-sighted, research suggests.

An analysis of eight previous studies by University of Cambridge researchers found that for each additional hour spent outside per week, the risk of myopia reduced by 2%.

Exposure to natural light and time spent looking at distant objects could be key factors, they said.

The studies involved more than 10,000 children and adolescents.

Researchers are presenting their findings at the American Academy of Ophthalmology annual meeting in Florida.

Dr Justin Sherwin and his research team concluded that short-sighted children spent on average 3.7 fewer hours per week outdoors than those who either had normal vision or were long-sighted.

But they said the reasons why were not yet clear.

They expected to find that children who spent more time outdoors also spent less time doing activities like reading, studying or playing computer games, but no such link was found in two of the eight studies which looked at this relationship.

However, Dr Sherwin said they would now need more precise data to try to understand which factors, such as increased use of distance vision, reduced use of near vision, natural ultraviolet light exposure or physical activity, are most important.

There are also other factors to consider, he said.

"Any increase in time spent outdoors must be weighed against exposure to UV radiation - and the increased risk of skin cancer, cataracts and other cancers.

"On the other hand, increasing outdoor physical activity could protect against diabetes and obesity, vitamin D deficiency and osteoporosis, for example," he said.

Short-sightedness is a common eye condition that causes distant objects to appear blurred, while close objects can be seen clearly.

Myopia is the medical term for short-sightedness.

It is much more common today in the UK and the United States than it was just 30 to 40 years ago. Approximately 1-2% of five- to seven-year-olds in the UK have myopia.

About five million British people are short-sighted and some 200,000 of them will be seriously short-sighted.

In some parts of Asia, more than 80% of the population suffers from short-sightedness.

Short-sightedness results from excessively long growth of the eyeball, or a steeply curved cornea.

Dr Susan Blakeney, optometric adviser at The College of Optometrists, said children were normally born long-sighted.

"As they grow they become less long-sighted so that by the time children stop growing their eyesight should be perfect.

"If a child is not born long-sighted enough then they will overshoot and end up short-sighted. This tends to happen around puberty.

"There are numerous factors which could influence that journey - the question is what is the key bit that really makes a big difference."