Final answer:
REM sleep is often referred to as paradoxical sleep because of its combination of high brain activity and lack of muscle tone.
Step-by-step explanation:
REM sleep is often referred to as paradoxical sleep because of its combination of high brain activity and lack of muscle tone. During REM sleep, the brain waves resemble those observed when a person is awake. However, muscle systems in the body, except those involved in circulation and respiration, are paralyzed. This stage of sleep is also associated with dreaming. REM sleep is marked by rapid movements of the eyes under closed eyelids.