Self-control relies on glucose as a limited energy source. When we engage in acts of self-control, our glucose levels are depleted, making it difficult to exert self-control again until more glucose is consumed. Glucose is the primary fuel for the brain and its executive functions that require control and effort. Low glucose levels impair these controlled processes. Additionally, small acts of self-control are enough to deplete remaining glucose, impairing further control until more fuel is consumed. The strength model of self-control compares it to a muscle that gets tired with use over time. Like a muscle, self-control can be improved with practice but becomes vulnerable after exertion until "resting".