AP Psychology Practice Test 2025 - Free AP Psychology Practice Questions and Study Guide

The distinguishing feature of REM sleep is that, during this stage, while the body's muscles are significantly relaxed and exhibit a state similar to paralysis, other body systems, particularly the brain, are highly active. This paradoxical state is characterized by heightened brain activity that resembles wakefulness, which is associated with vivid dreaming. The body's total muscle atonia prevents the acting out of dreams, allowing for this unique combination of physical inactivity and cognitive activity.

The other options do not accurately describe what sets REM sleep apart. The presence of dreams is actually more prominent during REM sleep, and while memory consolidation is an important process that occurs during sleep, it is not exclusive to REM sleep; it also occurs in non-REM stages. Additionally, while heart rate and blood pressure can vary during different sleep stages, the unique combination of active brain function and muscle atonia is what specifically characterizes REM sleep.