This may explain the common nightmare of trying to run away from danger and being rooted to the ground. The signals that activate the eye muscles come from higher in the brain and are spared this paralysis. Jouvet, in the 1960s, operated on cats and cut the connection between the brain's inhibitory center and the spinal cord, so that muscle-activating signals were no longer blocked. During REM sleep these cats would stand up, arch their backs, and engage in fights with the figments of their dreams. Occasionally, human patients will behave like these transected cats, acting out their dreams, and actually inflict damage on those nearby.
Babies sleep much more than adults, and a much larger fraction of their sleep is REM sleep.
This suggests that REM sleep serves some important function in growth and development, but intensive study has so far failed to reveal what that function is. A standard scientific method for determining the function of an organ or process is to remove it and see what happens. Dement showed that when normal individuals are deprived of REM sleep for several days by being awakened in the sleep lab as soon as rapid eye movements begin, they will become anxious and irritable and have trouble concentrating. As soon as the interference stops, these individuals will go right into REM sleep without the usual 90-minute delay, and make up to some extent for the deprivation. This certainly implies that REM sleep is essential, but people have been deprived of it for as long as ten days without any serious effects.