Decision fatigue, a phenomenon increasingly intriguing psychologists today given the growing culture of endless distractions and over-stimulation, is a term coined by social psychologist Roy F. Baumeister. Vaguely introduced by Freud, he believed people constantly struggle to manage their inner desires against the external pressures of the ego, which in turn requires mental energy. Willpower is much like a muscle and when fatigued through overuse or when the brain is deprived of the energy necessary to sustain willpower, e.g. glucose etc — your willpower wanes.
Every decision you make, even insignificant ones like whether to have lemonade or water, and every temptation you avoid depletes your willpower.