"Think of willpower like the power bar on your cell phone. Every morning you start out with a full charge. As the day goes on, every time you draw on it you’re using it up. So as your green bar shrinks, so does your resolve, and when it eventually goes red, you’re done. Will power has a limited battery life but can be recharged with some downtime. It is a limited but renewable resource".
"Everyone accepts that limited resources must be managed, yet we fail to recognise that willpower is one of them. We act as though our supply of willpower were endless. As a result, we don’t consider it a personal resource to be managed, like food or sleep. This repeatedly puts us in a tight spot, for when we need our willpower the most, it may not be there".
- Gary Keller - p.65 of The One Thing -