Complex sentence: REM stands for "rapid eye movement" but involves many other aspects including virtual paralysis of thean anabolic state, building up the immune, nervous, skeletal, and muscular systems.
Split into simpler sentences: REM stands for "rapid eye movement" but involves many other aspects including virtual paralysis of the body. During sleep, most systems in an animal are in an anabolic state, building up the immune, nervous, skeletal, and muscular systems.