My buddy Jake had the same problem after a gaming marathon. He started setting a timer on his phone to get up every thirty minutes, even if it's just to walk to the kitchen and back. He'll do a couple of lame shoulder rolls or just stand and stretch his arms over his head. Seems silly but he says it totally fixed his next-day ache.
My cousin Mark tried the timer thing like @gibson.holly's buddy and said it completely wrecked his focus. I mean, getting pulled out of the zone every half hour just made him more annoyed and he never really got back into the game. Maybe it's just me but that kind of forced break feels like it would make the whole session less fun. I'd rather just deal with a little stiffness later than have my night constantly interrupted.