There was one obvious culprit: book work. The modern rise in myopia mirrored a trend for children in many countries to spend more time engaged in reading, studying or — more recently — glued to computer and smartphone screens. The average 15-year-old in Shanghai now spends 14 hours per week on homework, compared with 5 hours in the United Kingdom and 6 hours in the United States. Researchers have consistently documented a strong association between measures of education and the prevalence of myopia.

However, after studying more than 4,000 children at Sydney primary and secondary schools for three years, researchers found that children who spent less time outside were at greater risk of developing myopia. Close work might still have some effect, but what seemed to matter most was the eye’s exposure to bright light. The leading hypothesis is that light stimulates the release of dopamine in the retina, and this neurotransmitter in turn blocks the elongation of the eye during development. Researchers now suspect that under dim (typically indoor) lighting, the cycle is disrupted, with consequences for eye growth.

Based on epidemiological studies, Ian Morgan, a myopia researcher at the Australian National University in Canberra, estimates that children need to spend around three hours per day under light levels of at least 10,000 lux (someone under a shady tree, wearing sunglasses, on a bright summer day) to be protected against myopia. He recognizes that many schools do not have the flexibility to add time outdoors. So last year, in collaboration with Congdon, he began piloting the idea of teaching kids in a classroom made of glass to let in more natural light.

In some places, children cannot get any more outdoor light: there are too few hours of daylight, the sun is too fierce, or the cold too intense. Animal research10 has suggested that powerful indoor lights could do the trick instead: light boxes currently sold to treat seasonal affective disorder, for example, can deliver up to 10,000 lux illumination, but their effects on myopia have not been tested extensively in humans

Meanwhile, researchers have been working on ways to prevent myopia from worsening. Sankaridurg and her colleagues have developed special glasses and contact lenses that can alter eye growth by focusing light from distant images across the entire field of view, rather than just at the centre, as standard lenses do. Other research groups have shown that nightly eye drops with a neurotransmitter-blocking drug called atropine can also help to control myopia progression14, although the mechanism remains unclear. “

But eye drops and light boxes do not have quite the appeal of sending children outside to play, which has plenty of other benefits besides those for the eyes. “It probably also increases physical activity, which decreases likelihood of obesity and enhances mood,” Rose says. “I can only see it as a win — and it’s free.”