What is the cause of myopia? Researchers in a country tracked more than 500 children aged 8-9 with normal vision. Five years later, they found that the only environmental factor related to myopia is the time children spend outdoors. Moreover, children's myopia is indeed related to the length of time they spend outdoors, whether they are exercising, having a picnic or just reading on the beach. 
Those children who spent more time outdoors did not reduce the time they spent reading books, watching screens or studying closely. Some children spent very long time outdoors reading books and electronic screens.
 the researchers believe that, The more important cause of myopia may be whether the eyes are in bright light, which can protect the eyes.