In the post The Best Argument Against Free Will, I described the scientific case against the illusion of free will. Willpower is an idea often confused with free will. Willpower exists, but free will doesn’t. It’s an important distinction to make.
Willpower has a biological basis that involves blood glucose:
Self-control relies on glucose as a limited energy source: willpower is more than a metaphor. (Link)
Self-control without a “self”?: common self-control processes in humans and dogs. (Link)
Sweetened blood cools hot tempers: physiological self-control and aggression. (Link)
The physiology of willpower: linking blood glucose to self-control. (Link)
Toward a physiology of dual-process reasoning and judgment: lemonade, willpower, and expensive rule-based analysis. (Link)
Understanding impulsive aggression: Angry rumination and reduced self-control capacity are mechanisms underlying the provocation-aggression relationship. (Link)
Scientific research describes more constraints on human willpower in studies like these:
2. self control
Techniques exist to increase willpower, but finding information about these techniques and putting them into practice is itself determined by biological and environmental influences outside of an individual’s control. Acting on impulse or resisting temptation is determined by the condition of the brain at a given time.