Originally posted by mr.tajny:

For me its often motion blur, low FOV, walking effects, some kinds of antialiasing, etc, what causes headache and simillar. Try to turn all of them off and if it helps turn one by one back on to find out the one that is yours;)

also you might try: if you have a light source near your monitor turn it off and get indirekt light from the back - otherwise your eyes might a bit overstressed from the everyday work