"It is as if the mind is a terribly designed airplane. As long as the plane flies in a straight line, it burns very little fuel, but as soon as the pilot takes over in any way, to dive or bank or climb, the plane burns fuel at an alarming rate making it more difficult to steer in the future. At some point, you must return the plane to autopilot until it can refuel or else it crashes. In this analogy, taking control of the human mind includes making choices, avoiding temptation, suppressing emotions and thoughts, and acting in a way deemed appropriate by your culture. Saying no to every naughty impulse from raiding the refrigerator to skipping class requires a little bit of willpower fuel, and once you spend that fuel it becomes harder to say no the next time. All of Baumeister’s research suggests self-control is a strenuous act. As your ego depletes, your automatic processes get louder, and each successive attempt to take control of your impulses is less successful than the last. Yet, ego depletion is not just the effects of fatigue. Being sleepy, drunk, or in the middle of a meth binge will certainly diminish your ability to resist pie, but what makes ego depletion so weird is that the research suggests the system affected by lack of sleep and excess of drink can get worn out just from regular use. Inhibiting and redirecting your own behavior in any way makes it more difficult to delay gratification and persevere in the face of adversity or boredom in the future."
Read more about Ego Depletion here