Poisons make you feel very tired
Movement causes your muscles to contract. At the same time, your body produces lactic acid and other waste products, which act like poisons. When the blood carriers this “Poisons” to the different parts of the body, you feel tired. This type of internal poisoning, however, is necessary because your body needs to feel tired, in order to want to rest. During rest, these waste products are removed, and your body and brain recover.