For primary school children in China, spending an extra 45 minutes per day outside in a school activity class may reduce the risk of nearsightedness, or "myopia," according to a new study.

In some parts of China, 90 per cent of high school graduates have nearsightedness, and rates are lower but increasing in Europe and the Middle East, the authors write.

"There were some studies suggesting the protective effect of outdoor time in the development of myopia, but most of this evidence is from cross-sectional studies [survey] data that suggest 'association' instead of causality," said lead author Dr. Mingguang He of Sun Yat-sen University in Guangzhou. "Our study, as a randomized trial, is able to prove causality and also provide the high level of evidence to inform public policy."

Intense levels of schooling and little time spent outdoors may have contributed to the epidemic rise of nearsightedness in China, he told Reuters Health by email.

The researchers divided 12 primary schools in China into two groups: six schools continued their existing class schedule, while six were assigned to include an additional 40 minutes of outdoor activity at the end of each school day. Parents of children in the second group were also encouraged to engage their children in outdoor activities on the weekends.

In total, almost 2,000 first-graders, with an average age of almost seven years old, were included.

After three years, 30 per cent of the outdoor activity group had developed nearsightedness, compared to almost 40 per cent of kids in the control group, according to the results in JAMA.

That means kids who spent more time outside were 23 per cent less likely to develop nearsightedness, the authors write.

The study doesn't investigate why time outdoors might protect eyesight, but He said that some experimental work suggests brighter outside light affects eye growth in a way that inhibits myopia.

To maximize the benefit, schools in China should increase outdoor time further, including recesses, and encourage parents to bring children outdoors over the weekend, he said.

"One issue that needs to be addressed is the potential skin and eye damage from UV exposures, but these can be managed by standard UV protection measures," He said.

"Future studies should include information about the content of the additional outdoor activity, if the activity could be standardized, and how it differs from other studies," Dr. Michael X. Repka of the Johns Hopkins University School of Medicine in Baltimore writes in an accompanying editorial.

"Establishing the long-term effect of additional outdoor activities on the development and progression of myopia is particularly important because the intervention is essentially free and may have other health benefits," but parents should understand that the magnitude of the effect may be small and may not be permanent, he writes.

Spending time outside also involves focusing on more distant objects, one assumes, which may play a role in addition to sunlight exposure, Repka told Reuters Health by email.

"The effect is very modest and should not be overstated," he said.