When you start to feel that headache from the game, stop. Or put a time limit or something. I myself only played HL2 for an hour and then I just stopped and never touched it again (Due to system problems and stuff). But I usually put an hour when I stop playing (instance, goto sleep at 11:00) W/e works for you. Drinking water usually when ur playing a game helps u bring u back to reality for a good 4 seconds.