Outdoor play 'good for the eyes'
Getting children to play outside for 40 minutes a day could be a way to curb growing rates of short-sightedness, according to Chinese researchers.
They asked six schools to test the strategy over three years and it appeared to be beneficial.
The findings in Jama support the theory that children need to balance "close up" work, like reading, with activities that use distance vision.
Experts say although myopia is now very common, the cause remains unknown.
Short-sightedness or myopia is thought to affect up to one in three people in the UK and is becoming more common.
In myopia, the eye is unable to focus in the normal way which makes objects in the distance appear blurred.
It runs in families but environmental factors, such as spending lots of time on a computer or reading, have also been linked to the condition.
This has led researchers to question whether changing a person's early environment might cut their risk of myopia.
Dr Mingguang He and colleagues recruited 12 primary schools in China to take part in a three-year-long study to test this.
Six of the schools were asked to timetable a compulsory 40-minute session of outdoor play each day, while the other six stuck to their usual classes.
The children and their parents were also asked to keep a diary of how much outdoor play time they clocked up on weekends - this did not differ between the two study groups.
The researchers then set about testing the schoolchildren for any signs of myopia. At enrolment, fewer than 2% of children in each group had myopia.
Over the course of the study, 259 children out of 853 (30%) in the intervention group and 287 out of 726 (40%) in the control group were judged to have myopia - a refractive error of at least minus 0.5 Diopter on an eye exam.
Although this percentage difference is not huge, it is significant, say the researchers. And it remains even when you take into account other factors, such as family history of myopia.
"This is clinically important because small children who develop myopia early are most likely to progress to high myopia, which increases the risk of pathological myopia. Thus a delay in the onset of myopia in young children, who tend to have a higher rate of progression, could provide disproportionate long-term eye health benefits," the researchers say in Jama.
In an editorial in the same journal, Michael Repka from Johns Hopkins University, says more work is needed to confirm and understand the findings.
It may be that spending time outdoors limits how much time is spent doing "close up" activities, or that getting more daylight helps with eye growth and function, he says.