The feeling of being hangry -- that angry, annoyed, irritated sensation you get when you're hungry -- is real and can have a substantial impact on willpower. Research has proven that low blood sugar often weakens a person's resolve, making you grumpy and pessimistic.

When you're hungry, your ability to concentrate suffers and your brain doesn't function as well. Your self-control is likely weakened in all areas, including diet, exercise, work and relationships. So fuel up with healthy snacks and regular meals to keep yourself in check.

According to a study by Stanford University , the amount of willpower a person has is determined by their beliefs. If you believe you have a limited amount of willpower, you probably won't surpass those limits. If you don't place a limit on your self-control, you are less likely to exhaust yourself before meeting your goals.