I find that if I spend considerable time reading my eyes don't focus well on distant things. In my case I'm assuming it's because my eyeball is lengthened (something I evidently inherited from my Mom's grandmother) and the muscles don't respond as quickly as they had when I was younger. My eyesight is gradually getting where I need glasses for distant visions. But since I had my cataract surgeries with lens implants in 1999, I had vision without needing glasses for about 12 or so years.