This is probably due to the fact that the clock in the brain is stopped during sleep. When the body is at rest, the autonomous nerve system takes over. This means that basic functions continue (you would not want to stop breathing for example) but the body itself has switched of certain functions (they are not completely switched off of course but are dormant). One of these functions is a sense of time. One of the other functions is the influence of pressure on the skin. You can toss and turn (which is a way of preventing prolonged pressure on parts of the skin) without waking up or even being aware of it when you wake up.