Kelly McGonigal, PhD and author of The Willpower Instinct says willpower is a response that comes from both the brain and the body. The willpower response is a reaction to an internal conflict. You want to do one thing, such as smoke a cigarette or supersize your lunch, but know you shouldn't.

What is the science behind willpower?

Psychologists have found that willpower is a lot like stress: It's not just a psychological experience, but a full-blown mind-body response. ... The need for self-control sets into motion a coordinated set of changes in the brain and body that help you resist temptation and override self-destructive urges.

Is willpower really like a muscle?

Researchers who study self-control often describe it as being like a muscle that gets fatigued with heavy use. ... While muscles become exhausted by exercise in the short term, they are strengthened by regular exercise in the long term. Similarly, regularly exerting self-control may improve willpower strength.