We shouldn’t ignore any complicating factors, though. You could argue that perhaps parents who studied a lot themselves as children and ended up wearing glasses, are likely to encourage their children to do the same, making a genetic association appear stronger than it really is. Or perhaps some children inherit a susceptibility to eye problems, which is then activated by the strain they put on their eyes when they are young.
Donald Mutti and his colleagues in the United States tried to disentangle these complications in a study conducted in California, Texas and Alabama. They found no evidence of a genetic susceptibility and found that the children of parents with poor eyesight spent no more time staring closely at books than other children did. Heredity, the authors insist, was the stronger factor.
But returning to possible effects of the environment, there are a set of intriguing studies that have looked at the effect of light – not torchlight under the bedclothes, but bright daylight. Perhaps it’s not the time spent inside squinting at the page that’s the problem, but the lack of time spent outside. The Sydney Myopia Study followed more than 1,700 six and twelve year olds living in Australia and found that the more time the children spent playing outdoors, the less likely they were to have short-sightedness. A systematic review of studies including those from Australia and the United States found a protective effect overall of spending some time outdoors, particularly in East Asian populations.
Why could daylight help? There used to be an idea that playing sport taught children to focus on distant objects, but in this study it didn’t matter what they were doing while outside as long as they were out in daylight. This appeared to protect some children against the hours they spent reading or studying.
The authors believe that the benefits of being outdoors are less about looking into the distance, and more about the effect that daylight has on your depth of field and the ability to focus clearly. They even suggest that extra exposure to daylight could encourage the production of dopamine, which could then have an effect on eye growth. This hypothesis hasn’t been tested, but if it were to be demonstrated might it explain the low level of myopia in Australia.
With such a variety of papers on this subject with varied findings, what are we to conclude? Genetics undoubtedly has a major impact on rates of myopia, but evidence suggesting that environmental factors may have a role can’t be ignored just yet. After all, no matter how small an effect the environment has, it’s is easier to change than your genes.
The best we can say at the moment is that playing outside seems to be beneficial to the eyes and that perhaps young children should study in a good light to avoid straining their eyes. As for adults, all these studies were conducted on children whose eyes were still developing, so if you still want to read under the covers by torchlight then it’s unlikely to cause you any problems. Of course now that you’re old enough to decide your own bedtime, you probably don’t need to.
You can hear more Medical Myths on Health Check on the BBC World Service.
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