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Why is REM sleep sometimes called paradoxical sleep?

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REM sleep is referred to as paradoxical sleep because of the unique contrast between the brain's physiological state and the body's physical state during this phase.12

While the body is in a state of deep relaxation characterized by atonia—a temporary paralysis of skeletal muscles—the brain exhibits high levels of activity. Specifically, electroencephalography (EEG) recordings during REM sleep show low-amplitude, high-frequency beta waves that are similar to those observed during periods of wakefulness, rather than the high-amplitude, low-frequency waves typically associated with other stages of sleep.32

This combination of intense neural activity—often exceeding that of non-REM sleep and sometimes equaling or surpassing levels seen during wakefulness—occurring simultaneously with physical immobilization creates the "paradox" that gives the stage its name.42