I know this maybe unhealthy but I think it maybe interesting to some people from a scientific point of view.
I have been playing a game online for about 2 years now and I find if I play the game solidly for about 2 hours, my brain goes into an altered state, where I suddenly feel my brain speed up, and I become unaware of anything else in the room. In a sense, I actually "enter" into the game, but what I think is happening is that the brain *learns* to see the game in a new way. Bascially, the brain wires itself up to experience being in the game as opposed to seeing it on a monitor screen.
I guess this is why it feels like the brain is speeding up, because it starts using dedicated circuitry to deal with the information pouring in through the eyes and ears. In geeky terms, you could say my brain has developed its own co-processor, a bit like a hardware mpeg decoder, instead of software based decoder to handle the information processing part of game playing. It is a very strange experience.
I can't bring this state on at will, it's just a result of playing for a long time, but I rarely loose when it happens. The game is called Unreal.