The idea that getting outside is good for us is centuries’ old — but it is only recently that science has been able to unravel the multiple and surprising benefits, especially for our eyesight.
Short-sightedness, or myopia, is a condition which begins in childhood and isn’t simply a case of needing glasses, it is also a leading cause of adult-onset blindness. And the number of people developing myopia is increasing at an alarming rate.
In both the UK and the U.S., myopia rates have doubled since the Sixties and continue to rise. In Western Europe some 56 per cent of people are expected to be short-sighted by 2050.
Fact: Short-sightedness, or myopia, is a condition which begins in childhood and isn’t simply a case of needing glasses, it is also a leading cause of adult-onset blindness
Meanwhile, in East Asia the situation is even worse. They are grappling with an epidemic of the condition. In some urban areas, such as Guangzhou, the most populated region of southern China, the prevalence of myopia has reached 90 per cent, whereas 60 years ago just 10 to 20 per cent of the Chinese population was short-sighted.
Until recently the textbooks called it a genetic condition, but as Professor Ian Morgan, an expert in vision science from the Australian National University points out, the prevalence of myopia has been rising far faster than can be explained by natural selection.
Indeed, when he decided to investigate just how global a phenomenon this rise in myopia was, he discovered that, by the age of seven, the prevalence of it in Australia was 1 per cent, whereas across the water in Singapore it was 30 per cent. This difference could not be explained by genetics — among children of Chinese ancestry who had been raised in Australia the rate of myopia was just 3 per cent.
‘The only factor we could think of that differed, was the amount of time kids reported being outdoors,’ says Professor Morgan.
Further studies revealed that, whereas Australian children spend some four to five hours outdoors per day, in Singapore it was more like 30 minutes.
Professor Morgan’s theory that outdoor light might be protective was strengthened by animal experiments conducted at other laboratories.
For example, raising baby chickens in low light was found to significantly increase their chances of becoming myopic, and a separate study found that housing chicks in the equivalent of outside light protected them from developing an experimental form of myopia.
Furthermore, one study published in the journal PLoS One in 2014 found the most protective effects came not from exposure to continuous light, but to intermittent bright light — as we would to experience with the normal night and day cycle.
Theory: The theory is that, in the absence of bright daytime light, this rhythm becomes disrupted, resulting in retarded growth of the eyeball
Myopia is caused by the eyeball growing too long, resulting in the light from distant objects focusing a short distance in front of the retina (the layer of light sensitive cells at the back of the eye), rather than directly onto it. In severe cases, the inner parts of the eye stretch and thin, resulting in complications such as cataracts, retinal detachment, glaucoma and blindness.
The current best guess is that light stimulates the release of the hormone dopamine in the retina, which blocks the elongation of the eye among children (unfortunately, bright light doesn’t seem to reverse myopia in adults).
Release of dopamine in the retina is regulated by an internal clock and, as well as being needed for normal eye growth, it also acts as a chemical messenger allowing the eye to adapt to differing levels of light. Hence it usually ramps up during the day, enabling the eye to switch from night to daytime vision.
The theory is that, in the absence of bright daytime light, this rhythm becomes disrupted, resulting in retarded growth of the eyeball.
Ironically, given the impact that myopia can have on children’s education, it is the very desire to see their children excel at school that is contributing to the problem in East Asia.
The combination of intensive schooling and a lifestyle that actively discourages children from playing outside is depriving them of daylight — an essential ingredient of healthy eye development.
Our hypothesis would be that you need to be aiming for four or five hours of outdoor time each day, like Australian children have.
‘Also, children don’t go outside at recess time because they’re told it is bad for their skin,’ says Professor Morgan. ‘By contrast, in Australia it’s a punishment if you don’t go outside.’
The prevalence of myopia is also increasing in this country, where we have an iPad generation of youngsters who are more firmly wedded to staying inside with their gadgets than playing outside. Rates have in fact doubled since the Sixties and continue to rise.
Once myopia starts, it generally progresses until late adolescence. So if you can delay its onset by even a few years, it is hoped that this may drastically reduce the numbers suffering from severe myopia, with its associated risks. The solution may be relatively simple. In 2009, Professor Morgan launched an ambitious trial to put his theory about the protective effects of outdoor light to the test in Guangzhou.
At six randomly selected schools, six- and seven-year-old children would have a compulsory 40-minute outdoor class at the end of each school day.
The children were also sent home with activity packs containing umbrellas, water bottles and hats with an outdoor activity logo. They were rewarded if they completed a diary of weekend outdoor activities.
Children at six other schools carried on as normal, serving as controls. Three years later, Professor Morgan and his colleagues compared rates of myopia between the two sets of schools: in those with the outdoor intervention, 30 per cent of children had developed myopia, whereas in the control schools it was 40 per cent.
That may not sound like much, but the children of Guangzhou were only getting an extra 40 minutes of daylight per day, five days a week during term time — and from a very low starting point. Also, barely any families took up the challenge of venturing outdoors more frequently at the weekends.
‘Our hypothesis would be that you need to be aiming for four or five hours of outdoor time each day, like Australian children have,’ says Professor Morgan.
Another U.S. study found that spending ten to 14 hours per week engaged in ‘outdoor and sport activities’ was associated with approximately half the risk of developing myopia, compared to children doing less than five hours per week.
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