Final answer:
A defining characteristic of REM sleep is muscle paralysis, despite the presence of high brain activity similar to wakefulness, allowing for dreaming but preventing voluntary muscle movement.
Step-by-step explanation:
Characteristic of REM Sleep
A characteristic of REM sleep is muscle paralysis. REM sleep, otherwise known as rapid eye movement sleep, features brain waves very similar to those when a person is awake, which means there is high brain activity during this stage. It is during REM sleep that most dreaming occurs. Despite this brain activity, there is a paradoxical lack of muscle tone, preventing movement of the voluntary muscles, except for the muscles that facilitate circulation and respiration. This stage is marked by the characteristic rapid movements of the eyes under closed eyelids. In contrast, options suggesting increased heart rate, low brain activity, and slow eye movements do not describe REM sleep since heart rate can actually increase, brain activity is high, and eye movements are rapid.
Issues with REM sleep can lead to REM sleep behavior disorder (RBD), where the typical muscle paralysis does not occur, resulting in physical activity during REM sleep. However, this is a disorder and not representative of normal REM sleep.