I was getting double-vision when trying to read after watching YouTube videos on the iPod Touch for about 90 minutes. Text close-up with the reading glasses was difficult to read and my distance vision wasn't fantastic either. That was a bit of a worry, it's only been 12 months since my right eye had its cataract removed. That eye is supposed to have 20/20 vision, it was a long way from being that good.
So I went to bed in a darkened room for 45 minutes & now my vision is back to normal.
Phew.