I also work in IT and have found extended time in front of the computer to be challenging. My desk is also near a window, and on sunny days I can feel completely drained by 5pm. I'm actually sitting here waiting to do some after-hours work right now.

I found that sitting further away helps tremendously. What I've done on occasion is use my laptop as my keyboard/mouse, then keep my monitor plugged in but from a further distance (approx 4 feet for a 24" monitor). Something about the dim background seems to keep my eyes from feeling so strained.