Over the past few decades, the prevalence of myopia has exploded around the world. Scientists estimate that by 2050, half the world’s population will need glasses. For a long time, the high prevalence of myopia was thought to be related to spending too much time reading, watching television, or looking at phones. After all, reading and looking at screens can indeed cause visual fatigue, and it is easy to imagine myopia as the result of simply wearing your eyes out.
But in recent years, more and more evidence has found that books and screens may not deserve most of the blame for myopia (although they probably deserve some). Insufficient exposure to outdoor light is the more important risk factor. The amount of outdoor activity is strongly related to the prevalence of myopia.
(In Nature’s Healing (Da Ziran de Zhiyu), which I finished discussing in the earlier part of the New Knowledge Class, the author mentioned the relationship between outdoor activity and myopia. The book dealt with it very briefly, though, and I did not mention it in the program either. Here, I will draw on some other material I had read earlier to talk about the topic. The data below mainly comes from the March 2017 issue of Scientific American, in the article “Get More Sunlight to Prevent Myopia (Duo Shai Taiyang, Yufang Jishi),” by science writer Diana Kwon.)
The relationship between time spent outdoors and myopia was first discovered in two studies conducted in 2007. First, a U.S. survey found that after the factor of “time spent outdoors” was removed statistically, time spent reading was no longer related to the prevalence of myopia. Around the same time, another study compared Chinese children aged 6-7 in Sydney and Singapore. It found that although the Sydney children spent more time doing close-up work each day, their rate of myopia was only 8%, compared with 29% in Singapore. The main difference was their time spent outdoors: Sydney children spent more than 13 hours outdoors each week, while the Singapore children spent only 3 hours.
So what is the mechanism by which insufficient outdoor activity triggers myopia?
Research has found that insufficient light entering the eyeball can induce myopia in animals. Although the precise mechanism is not yet fully understood, evidence suggests that the retina releases dopamine in response to light stimulation. Dopamine then helps prevent the eyeball from elongating itself in order to see objects clearly, thereby inhibiting the development of myopia.
In short, sufficient light is the key: it promotes the release of dopamine from the retina.
And outdoor light is many, many times more intense than light in a typical indoor environment!
Data excerpted from Get More Sunlight to Prevent Myopia (Duo Shai Taiyang, Yufang Jishi):
“Indoors, light intensity is very low (a typical office or classroom provides around 200-500 lux). By comparison, even an overcast day outdoors is much brighter (up to 15 000 lux), while a sunny summer day is brighter still (up to 30 000 lux).”
These figures must run pretty counter to your intuition. Doesn’t it sometimes feel as if the outdoors on an overcast day is still darker than indoors with the lights on?
But that is actually a complete illusion. People have a powerful capacity to adapt to their subjective perception of light. In a dark environment, our eyes gradually adapt and “turn up” our subjective perception of brightness. The reverse is also true: after adapting to a bright environment, things do not feel quite so bright.
Your eyes may fool you, but your phone screen does not. In a room that subjectively feels quite “bright,” setting the phone screen to roughly medium brightness is usually enough to see it clearly. But the moment you step outdoors, even on an overcast day that feels pretty gloomy, you will immediately notice that the phone screen no longer seems bright enough. That is because outdoor illumination, even on an overcast day, far exceeds indoor artificial lighting; once outdoor light shines on the phone screen, it simply does not look bright enough anymore.
So how do we prevent myopia?
Very simply: spend more time outdoors.
A 2009 study in Taiwan found that during a one-year follow-up, only 8% of children who received an additional 40 minutes of outdoor break time every day developed myopia, compared with 17% of children in the control group. Around the same time, a study conducted in Guangzhou found that children who received an additional 40 minutes of mandatory outdoor time every three days had a 23% lower likelihood of developing myopia than ordinary children.
Scientists are also developing new study lamps with much greater light intensity than existing lamps (to simulate outdoor light intensity indoors). But until this technology matures, spending more time outdoors is probably the most feasible way to prevent myopia.
Looks like we have one more reason to get close to nature.
Although current research suggests that insufficient light intensity may be the most important risk factor for myopia, this mainly applies during the day. At night, with phone and computer screens everywhere, they may have to take some of the blame for myopia after all. Some research has found that melatonin secreted at night also affects eyeball development, and too much light stimulation at night (most of the time from screens) can change the way the eyes grow.
Taken together, a comprehensive approach to preventing myopia might be:
Spend more time outdoors during the day and reduce screen time at night (judging from the evidence accumulated so far, the former may be more important).
References:
1.Williams, F. (2017). The Nature Fix: Why nature makes us happier, healthier, and more creative. WW Norton & Company.
2. Diana Kwon & Hou Youyang (2017). Get More Sunlight to Prevent Myopia (Duo Shai Taiyang, Yufang Jishi). Scientific American, (3), 60-65.
Note
- This article in the “New Psychology Knowledge Mini-Class” series was written to accompany related content in Zhichao Wei: New Psychology Knowledge Class. It was a short popular-science essay published on the now-closed platform Fantuan, and the original link no longer exists.
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