Linda said:I don't see how the purpose of sleep paralysis is to stop you from acting out your dreams, because you are basically awake (not dreaming) in your mind, only your body hasn't followed. Rather, stopping you from acting out your dreams, is the purpose of the hormones and the parasympathetic nervous system, as they make your muscles not react to what's going on in your dreams. Sleep paralysis and its purpose, if it has one, is a phenomenon that I don't thind science has quite explained yet.
Sleep paralysis is most probably the same mechanism that keeps you immobile while you are actually sleeping and dreaming. In the majority of cases, the paralysis wears off before one wakes up, but in some instances it continues briefly even after the dreamer has awoken. Or at least, that's the explanation that seems to make the most sense.
The purpose of this paralysis, as stated already, is just to keep the dreamer from acting out his dream. Even under the influence of paralysis, motor signals are sent out to the actual body in accordance with what the dreamer does in his dream. This has been proven pretty conclusively via lucid dreaming, which is a type of dream where the dreamer becomes aware that he's dreaming and can consciously act in his dream world just as he would in the real one. From http://www.susanblackmore.co.uk/Articles/si91ld.html :
As we watch sleeping animals it is often tempting to conclude that they are moving their eyes in response to watching a dream, or twitching their legs as they dream of chasing prey. But do physical movements actually relate to the dream events?
Early sleep researchers occasionally reported examples like a long series of left-right eye movements when a dreamer had been dreaming of watching a ping-pong game, but they could do no more than wait until the right sort of dream came along.
Lucid dreaming made proper experimentation possible, for the subjects could be asked to perform a whole range of tasks in their dreams. In one experiment with researchers Morton Schatzman and Peter Fenwick, in London, Worsley planned to draw large triangles and to signal with flicks of his eyes every time he did so. While he dreamed, the electromyogram, recording small muscle movements, showed not only the eye signals but spikes of electrical activity in the right forearm just afterward. This showed that the preplanned actions in the dream produced corresponding muscle movements (Schatzman, Worsley, and Fenwick 1988).
Further experiments, with Worsley kicking dream objects, writing with umbrellas, and snapping his fingers, all confirmed that the muscles of the body show small movements corresponding to the body’s actions in the dream. The question about eye movements was also answered. The eyes do track dream objects. Worsley could even produce slow scanning movements, which are very difficult to produce in the absence of a "real" stimulus (Schatzman, Worsley, and Fenwick 1g88).
LaBerge was especially interested in breathing during dreams. This stemmed from his experiences at age five when he had dreamed of being an undersea pirate who could stay under water for very long periods without drowning. Thirty years later he wanted to find out whether dreamers holding their breath in dreams do so physically as well. The answer was yes. He and other lucid dreamers were able to signal from the dream and then hold their breath. They could also breathe rapidly in their dreams, as revealed on the monitors. Studying breathing during dreamed speech, he found that the person begins to breathe out at the start of an utterance just as in real speech (LaBerge and Dement 1982a).