The past few decades have seen a stunning rise in the incidence of near-sightedness, also called myopia. Mounting evidence points to children now spending most of their daylight hours in relatively dimly-lit classrooms. The same applies to homeschooling set-ups and remote learning during the pandemic lockdowns.
Interior office and school lighting measures only about 300 lux and the average home even less, whereas outdoor light intensity can measure 50,000 lux.
Just three generations ago, people got an average of 10 hours of daylight exposure every day. Today’s kids—and the rest of us—are relegated to relatively dim light for the bulk of our waking hours. This can have serious health consequences.
The reduction in daily light exposure over the years compared to that enjoyed by earlier generations coincides with the rising incidence of near-sightedness. The COVID-19 pandemic kept school-aged kids confined indoors and focused on screens, which further sped up the worldwide trend toward worsening eyesight. By contrast, increasing time spent outdoors reduces the chance that a child will become myopic.
Myopia typically starts during elementary school years, an age at which kids first need glasses. In this susceptible population, the growing eye elongates too much along its front-to-back axis. It is not possible to shrink the eye, so prescription glasses or contact lenses are necessary. Laser surgery is sometimes possible.
An elongated myopic eye is distorted such that it causes light to end up focused short of the retina. Happily, regular exposure to outdoor daylight of more than 10,000 lux regulates the growth of children’s eyes. More time on the playground might also dent another rich-world problem: childhood obesity.
Genetics play a part in who develops myopia. A child with two near-sighted parents has a 60% chance of also being near-sighted, whereas spending just two hours a day outdoors effectively neutralizes that genetic risk. How does this work? Bright sunlight stimulates dopamine release from a class of specialized retinal cells not involved with vision. These then trigger a cascade of chemical signals that retard the elongation of the eyeball.
In the 1960s, myopia was uncommon in East Asia. Now it is ubiquitous. In Hong Kong, Singapore, and Taipei, more than 80% of schoolchildren are now near-sighted. In the US, a study from California put the rate at 59% among 17- to 19-year-olds. Back in the 1980s, the Asian military noticed a big problem of an increasing number of its conscripts needing glasses to focus on distant targets.
A 1983 study confirmed that 70% of Taiwanese school graduates needed glasses to aim properly. The most recent rate in Seoul is 97%. Why such high rates in Asia? Its cultural stress on education leads to long school days and little time for sunshine. Private tutoring after school can last long after the sun has gone down.
Human trials back these observations. In a 2020 Taiwan study, primary-school students took part in a program that placed them outdoors for two hours a day. Evidence from Taiwan shows that giving primary-school children more time outdoors cuts the number of pupils who go on to develop myopia.
Myopia rates steadily declined between 2010 to 2015, reversing decades of rising rates. Outdoor exposure measures about 10,000 lux, whereas indoor levels rarely exceed 1,000 lux. Sunlight in the tropics can exceed 100,000.
Short-sightedness is hardly benign. Glasses and contact lenses are a lifelong and expensive inconvenience. In poor countries, parents simply cannot afford them. Laser corneal surgery, marketed as Lasik and other brands, is likewise expensive and does not always fix vision correctly. But the main worry is that severe myopia predisposes a near-sighted person to other middle-age eye diseases, some of which can lead to untreatable vision loss, such as macular degeneration.
Myopia is typically regarded as a curse of the bookish. The higher one’s participation in after-class activities, the more likely you are to be myopic. Education, obviously, is a proxy for something else: exposure to daylight. Studies in California and Sydney, Australia, found that time outdoors was strongly associated with a lower risk of near-sightedness. The activity didn’t matter—walking, sports, picnics—simply being outdoors was the crucial point.
The daylight hypothesis is corroborated by animal studies in which light levels can be carefully controlled: Dimness reliably produces near-sighted animals. And how might bright/adequate light work its magic or have its effect? Via dopamine, which in the retina helps regulate the rate at which the eyeball grows. Too little, and it grows too long to focus properly.