What is the primary characteristic of REM sleep?

The primary characteristic of REM (Rapid Eye Movement) sleep is indeed muscle paralysis coupled with active dreaming. During this stage of sleep, the brain is highly active, resembling wakefulness, while most skeletal muscles experience atonia, or paralysis. This paralysis serves as a protective mechanism, preventing individuals from acting out their vivid dreams, which are most frequent during this stage. The rapid movements of the eyes, alongside the high brain activity, indicate that the brain is engaged in processing information and emotions, contributing to the dreaming experience.

In contrast, other stages of sleep, such as deep sleep (non-REM sleep), are marked by lower brain activity and are more restorative. Dreaming can occur in these phases, but it is generally less vivid and memorable. Therefore, the unique combination of active dreaming and muscular atonia makes REM sleep distinct from other sleep stages.