Think about how you feel after a long, busy day. After running errands, working on projects, and rushing to appointments, do you still feel like you have the energy to work on your goals? Once the day was done, you probably want to collapse in front of the TV and avoid doing anything at all.
People having a high level of self-control experience better relationships and have higher achievement levels. Lack of self-control is associated with social conflict and low-grade academic performance.
Example: While following a diet regime, a person spends a lot of willpower the whole day trying to avoid junk food, but by the end of the day, all his mental energy has been exhausted, and there is no self-control left, resulting in snacking on unhealthy food.