It is believed that sleep paralysis is caused by a disturbed REM cycle during awakening or falling asleep. Throughout the course of normal sleep, the body persists in a state of safe paralysis where only shallow breathing and eye movements are present. However, sometimes the brain awakens during the REM cycle while the body is still paralyzed. In this state one is also highly susceptible to hallucination, which, along with not being able to move or talk, can create very vivid and often terrifying experiences where one sees or feels an entity sitting on one's chest, making it hard to breathe and move.

It is also believed that this is merely an attempt to rationalize the various demons, ghouls and night creatures that prey on us at our most vulnerable. Which is why I always sleep with a Van Helsing emergency kit on my night stand.

Image created with Daz Studio, ZBrush and Photoshop.