more on dreams...

I found this in a textbook on biological psychology:

"The fact that our brains contain an elaborate mechanism whose sole function is to keep us paralyzed while we dream - that is, to prevent us from acting out our dreams - suggests that the motor components of dreams are as important as the sensory components. Perhaps the practice our motor system gets during REM sleep helps us to improve our performance of behaviors we have learned that day. The inhibition of the motor neurons in the spinal cord prevents the movements being practiced from actually occurring, with the exception of a few harmless twitches of the hands and feet."

Carlson, Neil. Physiological Psychology. Boston: Pearson, 2005.