Your Muscle are tight, you can not move. Most of us have had one of these dreams or someone has told you about the night they dreamed of being like stone.
One of the most troubling dream events, and startling physical side-effects of REM, is paralysis. Large muscle groups often become paralyzed during a dream, presumably to prevent injury to the dreamer in case the dreamer's instincts would cause a physical reaction to dreaming visuals.
It can be troubling if the dreamer becomes aware of his body in a paralyzed state without being aware of the fact that the mind is still in a dream state. Suddenly stripped of every physical capacity for defense, the dreamer can experience great panic or victimization in the dream.
This scenario is a troubling version of the lucid dream. Instead of mind awareness and body control, the dreamer has body awareness and no mind control. Hundreds of years ago, this phenomenon was observed and named having a witch on your back.
The idea was that an unfriendly spirit within the dream had pinned you to your bed. Indeed, it is not uncommon to have a sense of spiritual oppression in a dream that includes paralysis.