Great article by Kathy Sierra on how limited our cognitive resources are:

Spend hours at work on a tricky design problem? You’re more likely to stop at Burger King on the drive home. Hold back from saying what you really think during one of those long-ass, painful meetings? You’ll struggle with the code you write later that day. Since both willpower/self-control and cognitive tasks drain the same tank, deplete it over here, pay the price over there. One pool. One pool of scarce, precious, easily-depleted resources. If you spend the day exercising self-control (angry customers, clueless co-workers), by the time you get home your cog resource tank is flashing E.