Myopia, or nearsightedness, is reaching epidemic proportions. In the US, over 40 percent of the population needs glasses; estimates suggest that one-third of the world's population will be nearsighted by the end of the decade. Myopia can usually be treated easily with glasses, contact lenses or surgery, but even still nearsighted people are at a higher risk for glaucoma, retinal detachment, and cataracts. The condition's sheer pervasiveness has made researchers wonder about its cause. Now, scientists think they have a good understanding of why the condition has become more common: young people are spending too much time indoors, according to a report published today in Nature.