REM (Rapid Eye Movement) sleep is often referred to as "paradoxical" sleep due to its unique combination of physiological characteristics that seem contradictory.
During REM sleep, the brain exhibits patterns of activity that closely resemble wakefulness—active brain waves, increased heart rate, and irregular breathing—while the body experiences atonia, a temporary state of muscle paralysis that prevents individuals from acting out their dreams. This paradox arises because, although the brain is highly active and dreaming occurs, the body is effectively immobilized.
In summary, REM sleep is considered "paradoxical" because, despite the high level of brain activity and vivid dreaming, the body is unable to move, creating a contrast between the mental and physical states.