The limits of self-control are key factors in our poor eating habits. One reason self-control is limited is because the capacity of the brain’s information-processing system is relatively minuscule.

Scientists generally agree that our brain has two operating systems: a cognitive system and a noncognitive system. The cognitive system requires conscious awareness; it is reflective and deliberate. It can perform mathematical computations, make novel decisions, and engage in long-term planning and “out-of-the-box” thinking. It operates, on average, less than 5 percent of the time and is the internal resource responsible for self-control.

The other 95 percent of the time, our noncognitive system is in control. Impulsive and immediate—and following well-established rules and patterns—it is responsible for quick, automatic decision-making.

This has a lot of explanatory power. 

It also speaks to decision fatigue destroying willpower.