I hate when this happens.
Am I the only one that has never owned a pair of sunglasses (ignoring a pair of mini Ray Bans I had when I was ~4 years old)? I understand that there are applications for people that drive frequently towards the sun's light, but just walking around I've never understood it. Plus it means that if you say hi to me wearing them that I'll have no clue who you are.
My eyesight is perfect. Considering all the people I know that wear sunglasses and need contacts, I hereby theorize that sufficient doses of ultraviolet light are necessary for properly exercising the various optic muscles, and that without said exercise they will atrophy and be unable to exert sufficient motor force to bend the optic lens to the appropriate focus level.