Spending more time outdoors reduces a child’s risk of becoming short-sighted, academics suggest. For every hour spent outside, the chances of needing glasses drops by 2 percent, a review of previous studies shows. Overall, children who are short-sighted spend on average 3.7 fewer hours a week outdoors than those who had normal vision or were long-sighted. It is possible that greater exposure to natural UV light outdoors makes a key difference, along with more time spent looking at the horizon and distant objects. It is thought natural light, which is many times brighter than artificial light, can trigger the release of the brain chemical dopamine, which then stops the eyeball growing out of shape. The review cuts across the popular notion that it is too much time spent reading and on computers that causes short-sightedness, although experts say this may contribute to the problem. Its alternative theory is that time spent outdoors helps to protect eyesight against other risk factors. Short sightedness or myopia, which makes distant objects seem blurred, often begins in childhood and the teens.