Our brain uses two different and often conflicting systems to process information, drive our choices, and behaviors.
Mind #1: The logical mind: "I’ll stick to my diet because it is healthy and will help me lose weight." The challenge with the logic based mind it the fact that it is much harder to exercise willpower when fatigued or stressed. You may start the day with good intentions but end the day in a very different place. It is much easier to keep your long term goals in mind when making decisions when you are well rested.
Mind #2: The impulsive, emotional mind: The emotion-based mind is experiential and often automatic. It motivates us based on how we feel and is often outside our awareness. Logic is screaming at you to avoid the cookies. Before you know it, half the box is gone!
This phenomenon goes beyond diet and very much applies to exercise and other health behaviors. When we are in the moment, our choices are often driven by emotion especially when willpower is low. It is much harder to think about long-term goals and our future-self when we are low on energy.