It's like getting an oil change while the car is still running and being driven. The oil that is in circulation is at a high temperature so it's inefficient for lubrication and movement. The engine components are also moving at a high speed so any opening of the oil tank could cause a disruption to the movement of the car and could potentially cause damage. The same is true for sleep, while you are awake, your body is in a constant state of motion and as a result cannot receive the full benefits of quality rest and healing. To get the full restoring benefits, your body needs to power down and go into a restful state.