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When we go into rapid eye movement or REM sleep most of the muscles in our body are paralyzed so that we cannot move. The most common explanation you will hear from sleep scientists for this strange phenomenon is that the paralysis prevents us from acting out our dreams. I have never accepted this “explanation”.
If the REM atonia functioned to prevent dream enactment then why don’t we have atonia with NREM sleep? After all we have very complex, vivid dreams during NREM sleep as well. Sleep and dream scientists have demonstrated beyond any reasonable doubt that we experience complex, story-like and bizarre dreams in stage N2 NREM sleep. Indeed, we experience some forms of mentation throughout the entire sleep cycle--- even slow wave sleep! Yet the atonia only occurs during REM. Many dream scientists claim that the dreams that we experience in N2 NREM are indistinguishable from dreams we experience in REM sleep. Other dream scientists, myself included, believe that there are significant differences in content of REM vs NREM dreams but these content differences are unlikely to explain the REM atonia!
So if the presence of dreaming cannot explain the REM atonia what can? Why would Mother Nature paralyze the body during REM but not during the other sleep stages? The brain is quite active in REM but it is just as active in N2 NREM—indeed N2 is closer to the waking state than REM. So it cannot be brain activation levels that require the atonia.
So if the presence of dreams and brain activation levels do not explain REM atonia what can? I do not have an answer and I am unaware of any plausible hypotheses out there in the scientific literature.
Most scientists seem all too content with the idea that the atonia prevents dream enactment. They therefore do not even search for alternative explanations. In favor of the "atonia prevents dream enactment" story is the old experimental work by Jouvet and colleagues that showed when you abolished the cells responsible for atonia cats appeared to act out their dreams. In addition, REM Behavior Disorder is associated with loss of REM atonia and very clear dream enactment behaviors.
There is no doubt that when REM atonia is abolished you will often, though not invariably (not all RBD patients enact dreams) get dream enactment behavior. But of course, that does NOT prove that the dream enactment behavior is due to the absence of the atonia. Again you can have absence of atonia and not get enactment behavior. RBD patients do not evidence enactment behaviors every time they go into REM for example. Conversely you can get the opposite profile in narcolepsy: you can get REM atonia and still have dream enactment behaviors. Something besides mere loss of atonia causes the enactment behavior.
Regardless of what causes dream enactment behavior (it is not a mere release from inhibition phenomenon) we are no closer to understanding why REM atonia exists in the first place. It is not there to prevent dream enactments and has nothing to do with brain activation levels. Could it be related to other features of REM? For example, thermoregulatory reflexes decline during REM, autonomic nervous system discharges occur, sexual activation occurs, cardio-repiratory changes occur….but it is difficult to imagine why any of these features would require muscle paralysis. In short, REM atonia remains a scientific mystery-at least to me. I am not sure other sleep scientists would agree with me on this.
