The paralysis of sleep only occurs during the state of rapid eye movement (REM) sleep. REM sleep recurs about every 90 minutes and lasts an average of 15-20 minutes, but varies greatly depending on the time of night, increasing as morning approaches. During REM sleep, the motor cortex sends out a barrage of motor commands to the spinal cord, just as they do during waking movements. However, despite descending commands to move, motor neurons in the spinal cord are actively inhibited during REM by a set of local, glycine-containing inhibitory interneurons, preventing us from acting out our dreams and thereby temporarily and reversibly paralyzing us so that we do not act out our dreams.
Sleep apnea, where the airway becomes relaxed to the point of closing, causes many more brief arousals as the carbon dioxide (CO2) signals build in the bloodstream, are sensed by the brainstem, and the person awakens briefly to reestablish enough muscle tone to open the airway to breathe again. People with sleep apnea can have hundreds of such brief microarousals each night and not remember them, though they are accompanied by sometimes violent jerks.
Gina Poe, PhD, is an associate professor of anesthesiology, molecular and integrative physiology, and neuroscience at the University of Michigan. She has been conducting research in the field of basic sleep and consciousness since 1987 and in the role of sleep for learning since 1995, funded by the National Institute of Mental Health, the University of Michigan Department Of Anesthesiology, and the National Institute for Aging.
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