Willpower and cognitive processing draw from the same pool of resources.
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.

(via Your app makes me fat — Serious Pony )