"But everyone's self-control is a limited resource; it's like muscle strength: the more we use it, the less remains in the tank, until we replenish it with rest. In one study of self-control, participants first had to resist the temptation to eat chocolate (they had a radish instead); then they were given a frustrating task to do. The test was to see how long they would persist. Radish-eaters only persisted on the task for about 8 minutes, while those who had gorged on chocolate kept going for 19 minutes. The mere act of exerting willpower saps the strength for future attempts. These sorts of findings have been repeated again and again using different circumstances.

"We face these sorts of willpower-depleting events all day long. When someone jostles you in the street and you resist the urge to shout at them, or when you feel exhausted at work but push on with your email: these all take their toll. The worse the day, the more the willpower muscle is exerted, the more we rely on autopilot, which means increased performance of habits [especially bad habits or habits we are trying to change]. It's crucial to respect the fact that self-control is a limited resource and you are likely to overestimate its strength. Recognizing when your levels of self-control are low means you can make specific plans for those times."