One thing I discovered through an <i>ad hoc</i> anecdotal experiment involving only myself, over a quarter century ago, was the effect of font size. As a CS undergrad I was working like crazy in the lab for many hours a day. The display-hardware was B&W X window terminals (cathode ray, of course) with 19" screens. I had this idea: if I make things easier to read even when my eyes are tired, I can boost my endurance, right? So I switched to using large fonts. This had exactly the opposite effect; my fatigue symptoms became worse, because I was able to ignore the status of my eyes and keep going (without even consciously realizing it). If the fonts are big enough, you don't even have to blink any more; you can still read if your eyes dry out, ha! A that point I experienced an "aha" moment, and tried it the opposite way: I used a smaller font than the default. Like magic, my fatigue symptoms soon went away. I hypothesized that this is because small fonts create a negative feedback. When your eyes start to lose focus, you cannot read the screen any more. Firstly, you are forced to blink more often to keep your eyes moist. Second, you have no choice but to relax and not squint or strain. When the feedback mechanism does break, <i>you're</i> forced to take a break simply because you cannot ignore the fact that you're having trouble reading what's on the screen.<p>Since that time, I have always used small fonts and haven't had any problems. Plus, when CRT's started to disappear around the turn of the century, that was a fantastic change. Solid-state screens are much easier on the eyes.