When you move the muscles in your body, they produce a substance called sarcolactic acid, also called the “acid of fatigue.” When too much of this acid forms around a muscle, the muscle becomes “tired.”
The same sarcolactic acid is found in your blood too, as it travels to all parts of your body. So your entire body, and not just one muscle or set of muscles, becomes tired after a period of time.
When you sleep or rest, however, your body clears away these acids from around the muscles and from your blood, giving your body a chance to repair or replace cells, oil your joints, and recharge your brain and nerve cells with electrical energy.
