More and more children need glasses because they cannot see far things clearly. This is called myopia. Experts think about half of the people in the world may have myopia by 2050.

But there is some good news. In China, some schools gave children 40 more minutes outside every day. After three years, fewer children had myopia than at other schools. Playing outside may help protect children's eyes.

Myopia, or nearsightedness, is increasing quickly around the world. One study estimated that about half of the world's population, around 4.8 billion people, will be nearsighted by 2050. Myopia happens when the eyeball grows too long. Strong myopia can raise the risk of serious eye diseases later in life, so preventing it in childhood is important.

In a 2015 study in China, six primary schools added a 40-minute outdoor class every day, while six other schools did not. After three years, 30.4% of the children in the outdoor schools had become nearsighted, compared with 39.5% in the other schools. Researchers believe bright outdoor light may help stop the eyeball from growing too long.

Myopia is spreading so rapidly that a 2016 analysis projected it would affect about 4.8 billion people, or nearly half of humanity, by 2050, up from about 1.4 billion in 2000. The condition occurs when the eyeball grows too long, and high myopia increases the later risk of retinal detachment and glaucoma. A simple change at school seems to help.

In a cluster randomized trial published in 2015, twelve primary schools in Guangzhou, China, were divided into two groups. Six added a daily 40-minute outdoor class for first graders and encouraged outdoor play at weekends; the other six continued as usual. After three years, 30.4% of children in the intervention schools had developed myopia, compared with 39.5% in the control schools. A Taiwanese program sending children outside during recess also reduced myopia onset.

Earlier Australian research found that outdoor time was linked to less myopia regardless of how much near work children did. Bright light, not just distant viewing, may be what protects. The leading explanation is that light triggers dopamine release in the retina, but this mechanism rests mainly on animal studies. Outdoor time also seems better at preventing myopia than at slowing it once it has begun.