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sleep paralysis[′slēp pə‚ral·ə·səs]
Sleep Paralysis (Sleep Immobility)(dreams)
Most people have had the experience of not being able to move in a dream. Being unable to run away from some kind of danger—or trying to run and being able to move only very slowly—is particularly common because at some level we know that we are paralyzed when we dream. During the rapid eye movement (REM) stage of sleep, when our most active dreams occur, a relay station at the top of the spinal column disconnects the motor cortex from the rest of the body, with the exception of the lungs and the eyes. This is why the neck muscles lose their tone during this stage, which is one of the defining characteristics of REM sleep.
Clearly this is a biological mechanism for preventing us from awakening otherwise we might thrash about during dreams. This disconnection of the motor impulses is the reason sleepwalking occurs only during non-REM sleep. It is also a factor in the sleep disorder referred to as sleep paralysis (which is distinct from normal REM sleep immobility), in which the sleeper is completely paralyzed immediately before entering (and sometimes immediately after leaving) sleep.