Programming and stress

i'm in the middle of making a fairly sophisticated game. Most of the time I've spent on it so far has been just on the planning, how to get the physics and collision detection to work etc. I can only spent so long a day trying to work this out before i start getting stressed and stuff. i start feeling easily agitated and restless and it gets hard to concentrate on the game again. you guys have any tips for tackling programming stress? much obliged 8]

Also, if you're too fat and unhealthy to take a two hour walk (as I suspect most posters to /prog/ will be), try public transport. It works just as well.