I think it's very common for people to use "wrong" settings on their monitors (besides often bad ergonomics in general). If you have pretty much any amount of natural ambient light in your office, and you don't adjust the monitor's brightness over the course of the day, it's probably wrong for long durations. Many folks seem to leave monitors at default settings, which are generally way too bright for most environments. A simple test would be holding a piece of paper up to the screen - white on the screen shouldn't be brighter than the paper.