Why has nearsightedness more than doubled in 50 years ?  

TreeHugger has a post on research into the increasing incidence of myopia - with lack of exposure to sunlight being a possible cause - Why has nearsightedness more than doubled in 50 years?.

Sixty years ago, 10 to 20 percent of the Chinese population was nearsighted, now up to 90 percent of teenagers and young adults have trouble seeing distance. In other parts of the world the story is similar: Half of the young adults in the United States and Europe now have myopia, double the number of half a century ago. And some are predicting that by 2020, one-third of the world’s population could be diagnosed with the condition. ...

But surprisingly, it’s not the reading and computers and smartphones that are to blame. Now researchers believe that it’s the very act of spending too much time inside that is causing the problem. After a great deal of research and eliminating other factors, scientists now think that it boils down to exposure to light. Regardless of what kids are doing – whether sports, or playing, and even those who continue to do “close work” (like reading) outside – what seems to be key is the eye's exposure to bright light.

So while our kids are losing their connection to nature – while they’re becoming increasingly unfamiliar with the feeling of grass underfoot, mud in the hands, the sound of birds, the smell of dirt – they’re also losing the ability to see.