Old Hag Syndrone
Victims of "Old Hag Syndrone" awake to find that they cannot move, even though they can see, hear, feel and smell. There is sometimes the feeling of a great weight on the chest and the sense that there is a sinister or evil presence in the room and are often quite frightened about what is happening to them. The name of the phenomenon comes from the superstitious belief that a witch - or an old hag sits or "rides" the chest of the victims, rendering them immobile.
Although that explanation isn't taken very seriously nowadays, the perplexing and often very frightening nature of the phenomenon leads many people to believe that there are supernatural forces at work - ghosts or demons. The experience is so frightening because the victims, although paralyzed, seem to have full use of their senses. In fact, it is often accompanied by strange smells, the sound of approaching footsteps, apparitions of weird shadows or glowing eyes, and the oppressive weight on the chest, making breathing difficult if not impossible. All of the body's senses are telling the victims that something real and unusual is happening to them. Rather than ghosts or demons, however, there is probably a scientific explanation: "sleep paralysis" or SP (sometimes ISP for "isolated sleep paralysis").