When the effort of the engine is mild it gives it time to completely burn all the fuel. Posture, Endurance, and Sleep are characterized by a complete burn of the fuel which requires oxygen and releases carbon dioxide. Imagine driving a car very conservatively. You can gradually increase your speed without having to floor it. The endurance range ends when, if going any faster, you will not able to completely burn the increasing amount of fuel entering the cell. Sleep tests the other end of the spectrum where you will turn the idle down as low as possible without the car dying.
