Children of school age who spend more time outdoors may be less prone to myopia, or nearsightedness, researchers in China are reporting.

The protection of eyesight joins other health benefits of playing outside, including improved physical condition and a decreased risk of obesity, they say.

Adding a daily outdoor activity class for three years at a school in Guangzhuo reduced the rated of myopia, a condition in which close-up objects can be seen in clear focus but distant ones can't, the researchers report in the Journal of the American Medical Association.

Nearsightedness has reached epidemic levels in some urban regions of Asia, the researchers note, with between 80 and 90 percent of high school graduates having myopia.

In the United States, estimates put the incidence of myopia in children at about 50 percent.

For their study, the researchers selected 12 schools and looked at around 1,900 first graders. At six of the schools, a 40-minute period of outdoor play and activities was added to the schedule.

After three years, 30.4 percent of the students who spent extra class time outside showed evidence of myopia, a reduction of 23 percent from the 39.5 percent in the control group not given the outside periods, the researchers found.

Any reduction, especially at an early age, is significant, because children who develop myopia at an early age are the most likely to have the condition worsen as they get older, they say.

"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 benefit," the researchers said in their published study.

While it's not exactly clear how outside activity can improve eyesight in children, there have been studies suggesting outdoor light with its higher intensity may increase dopamine levels in the eye.

Dopamine has been found to inhibit a kind of development in the shape of the eye associated with myopia, notes study author Dr. Mingguang He of Guangzhou's Zhongshan Ophthalmic Center.

The study is strong evidence of the benefit of outside activity on children's eyesight, he says.

"In fact, in order to maximize the benefit, we should further increase the outdoor time by using school recesses and encouraging parents [to] bring their children outside on weekends," He says.

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