Throughout the history of mankind there have been reports of people waking up to feel a sense of weight on their chests and an inability to move their limbs to escape this pressure. People who experience this type of phenomena almost unanimously describe a feeling of panic and fear at their experience which can last long after the paralysis of their bodies has faded and they can move again.
Legends have arisen in every culture as a means of explaining this sensation, now called sleep paralysis.
Modern scientific study has discovered a common feature with most of these sleep paralysis events and a physiological cause for the most pronounced symptoms. It has been discovered that during sleep the brain releases a hormone into the blood stream that will paralyze the major muscle groups to prevent locomotion. In this way, the body can rest while the mind is working its way through dreams. This hormone is designed to prevent the body from hurting itself during the unconscious sleep state. Those who have had too little of the hormone introduced are subject to limb thrashing and sleepwalking as the person acts out the events of their dreams.
The medical explanation of sleep paralysis is that as a person rouses from the deeper sleep cycle, the hormone will take a few moments to wear off and release the muscles of the arms and legs, thus causing a brief period where the paralysis lingers. Generally it only takes a few seconds for this to fade, but even such a brief time can cause panic in many people. That a sense of fear or dread accompanies this sensation can be understood in that one of the major triggers for sleep paralysis is extreme stress in one's life.
While the medical profession considers all the sensations of sleep paralysis to be psychosomatic, paranormal researchers have often encountered stories of being held down or choked in places that exhibit poltergeist activity. There have been other reports, some as recent as the early 1990's, of a person's spouse entering the room where their partner is experiencing "sleep paralysis" and seen a dark shadowy form fleeing the bed of the victim.
There has yet to be a practical way discovered to study the most physiological aspects of this phenomena. But there has also been the belief of malicious spirits bent on harming the living for as long as humans can remember.
Does the truth lie somewhere in between?
Could it not be equally valid that certain dark spirits use this physical effect in an effort to attack the souls of the living when they are in a defenceless position?