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.