[Chris Wild]

It is normal for the body to be paralyzed during REM sleep (the dream state). This is called atonia and is as I said, perfectly normal. This is what keeps people from acting on what they are dreaming. People who sleep walk do so because the natural paralysis that is supposed to happen during dreaming doesn't.

Being awake and paralyzed can certainly happen if things get out of whack, though is most likely to occur if the sleeper is suddenly awakened from a dream state and the neuro chemicals that cause atonia have not yet dissipated. This also explains some of the panic involved, because the brain may also be very aggitated from whatever the dream was and then the sudden awakening.

For it to occur on occasion, especially if there are a lot of external environmental occurrences that can disrupt REM sleep, isn't unusual. If it becomes chronic, seeing a doctor or sleep specialist would be in order.