Children who spent more time than others outdoors at age 8 or 9 were only about half as likely to become nearsighted by age 15, U.S. researchers say. Dr. Cathy Williams of the University of Bristol and Jez Guggenheim of Cardiff University and their colleagues tracked the occurrence of nearsightedness in more than 7,000 boys and girls participating in the Children of the 90s study at ages 7, 10, 11, 12 and 15. The researchers analyzed the time the children spent outside at age 9 and how much physical activity they engaged in at age 11. Time spent outdoors was measured by a questionnaire filled in by the children\'s parents and their physical activity was recorded objectively using an activity monitor they wore for a week at age 11. Children ages 8 and 9 who spent the most time outdoors were about half as likely to develop myopia by age 15. The protective effect of being outside was unrelated to whether the children\'s parents were nearsighted or how much time the children spent reading, Williams said. \"There is now a need to carry out further studies investigating how much time outside is needed to protect against nearsightedness, what age the protective effect of spending time outside is most marked and how the protective effect actually works, so,\" Williams said in a statement. The study was published in the journal Investigative Ophthalmology & Visual Science.