Well, on my dad's old P3 dell which is about 9 years old, the only reason I'd be sick is if I use full screen mode and the video is really choppy or if I use the computer for 3 hrs straight. But the hardware's old and it has a CRT monitor so that explains what's going on.
My newer PC, I don't get that feeling when just watching a regular video unless I do just use my computer for too long but I was only playing the FPS game for about 20 minutes and at most 1 hour before I shut down the studio.