Sometimes people awake to find out that they are paralyzed, but can still see, hear, feel, and smell their surroundings.  Old Hag Syndrome comes from the superstitious belief that a witch or old hag sits or rides on the chests of victims and making them immobile.  During this state, there is a heavy weight on the chest and can be a sense of an evil presence.  The experience can be frightening since victims can sense what is happening around them, but cannot move or run.  Victims often have difficulty breathing with the weight on their chest.  There are many accounts where victims are actually forced into sexual intercourse or molested during this state by entities.  The only scientific theory is that Sleep paralysis may be the cause.