From the American Academy of Ophthalmology

This meta-analysis has found a pooled 2 percent reduced odds of myopia per additional hour spent outdoors per week in children and adolescents up to 20 years old. This is equivalent to an odds ratio of 0.87 for each hour outdoors per day. These results, considered in conjunction with a systematic review of studies not meeting the meta-analysis criteria, suggest that increasing time spent outdoors could confer a modest, but significant, reduced risk of developing myopia and its progression. Ophthalmology, October 2012