Margen
952 first-year primary schoolchildren in Guangzhou, in southern China, went out into the playground for forty extra minutes every school day for three years. Nobody touched their homework or their reading hours. At the end of the three years myopia had appeared in 30.4% of them and in 39.5% of the other 951, who kept their usual timetable: nine percentage points of difference for going outside. The families in the first group had also been asked to take their children outdoors at weekends.
The trial was led by the ophthalmologist Mingguang He, ran from 2010 to 2013 and was published in JAMA, the journal of the American medical association, in 2015. It is a school-randomised trial, which is as close to a clean experiment as you can get with children: the twelve schools were drawn by lot into the two groups, so differences of neighbourhood, income or teaching staff spread themselves out.
We have been saying the same sentence at home for a hundred years. Don't read so close. Don't read in poor light. Put that screen away, you'll end up short-sighted. The belief did not come out of nowhere: in population studies myopia sticks closely to years of schooling and to near work, and where people study most, most people wear glasses. The correlation is real. What was wrong was the arrow.
The most elegant proof that it was wrong dates from 2008 and was not done by anyone in a laboratory: it was done by geography. Kathryn Rose and her team measured the eyesight of six- and seven-year-old children of Chinese descent in two cities, Sydney and Singapore. Same age, same ancestry, and two very different school systems. In Singapore 29.1% were short-sighted. In Sydney, 3.3%. Almost nine times fewer.
The Sydney children read more books a week, and did more near work in total, than the Singapore children. The factor that best explained the difference was the time they spent outdoors: 13.75 hours a week in Sydney against 3.05 in Singapore. Thirteen and three-quarter hours against three. The authors also point to the early educational pressure found in Singapore and not in Sydney.
The explanation that holds up best today is light, and intuition is off here by two orders of magnitude. A well-lit living room has some 300 or 400 lux, the unit that measures the light arriving at a surface. A cloudy day, outdoors, goes above 10,000. A sunny day, 100,000. In other words: sitting in the living room is not "almost like being outside": it is between twenty-five and three hundred times less light, depending on the day. The hypothesis is that this intense light triggers dopamine in the retina and that dopamine slows the lengthening of the eyeball, which is exactly what makes an eye short-sighted: an eye that is too long, in which the image forms before it reaches the back.
Myopia is not just a pair of glasses. A very elongated eye has a stretched retina for life, and that multiplies the risk of retinal detachment, of myopic maculopathy — the central patch that stops seeing — and of glaucoma when that child turns fifty.
Almost everything we know is about prevention, that is, about delaying the day it appears. Once myopia has started, being outdoors helps less to slow it down, and to slow it down there are real treatments — very dilute atropine drops, specially designed lenses — decided by an ophthalmologist and not by an article. And genetics weigh, and heavily. With two short-sighted parents your child's odds are high.
Aim for two hours of daylight, every day, and fit those two hours into the gaps that already exist and are not up for negotiation: walking to and from school, break time outside the classroom, the half hour after an afternoon snack. No sport or direct sun is needed: the shade of a playground at midday already has ten or twenty times more light than your living room, so a chair on a terrace with a book will do. In winter, use the middle of the day rather than the late afternoon. And ask for an eye test before school starts and then one a year, because a child who is beginning not to see at a distance does not complain: they move closer, they squint, and their marks drop without anyone knowing why.