Three reasons: it sounds reasonable, because the eye muscles are probably not 'designed' for prolonged use in short distance focus. Second, I've seen similar t

Three reasons: it sounds reasonable, because the eye muscles are probably not 'designed' for prolonged use in short distance focus. Second, I've seen similar things mentioned from other sources, which also seemed credible. Third, my experience - I can't abuse my eyes the way I did twenty years ago, and the "pause and look at a distant thing" seems to help. Actually, I prefer 20 yards/meters or more, instead of just 20 feet.