This is something pretty odd I noticed within 48 hours or owning my iPhone 5s. Compared to my iPhone 5, the screen is a tad warmer, but my eyes felt like they had to focus more. This, as well as pressure behind the eyes while watching videos or reading text is making me wonder if it's the screen quality(or lack thereof) causing it. I already wear glasses and don't have these issues looking at any other monitor/TV that I can recall. A lot of people were reporting dizziness from iOS 7 animations, but I loaded it on my iPhone 5 right after the GM version was released and never had these issues...just the eyestrain/dull headache/pressure behind eyes after purchasing a 5s. Increasing the boldness/contrast in accessibility did not help either.