Exercise actually draws energy from your body. You will start to sweat and breathe heavily as your body tries to intake more oxygen to support the amount of energy needed. After exercising, people can be left feeling exhausted and hungry. The reason you will feel hungry is because you get energy from food. Calories are measured by how much energy an item will give. You will also know that you will start sweating. This is energy being turned into heat because of all the movement.