**Core Concept**
REM (Rapid Eye Movement) sleep is a stage of sleep characterized by rapid eye movements, low muscle tone, and vivid dreams. It is one of the two main stages of sleep, the other being NREM (Non-Rapid Eye Movement) sleep. During REM sleep, the brain's neurotransmitters are active, and the brain processes and consolidates memories.

**Why the Correct Answer is Right**
The correct answer is that **dreaming occurs during REM sleep**. This is because the brain's neurotransmitters, such as norepinephrine, serotonin, and acetylcholine, are deactivated during REM sleep, allowing the brain to process and consolidate memories in a more creative and abstract way, resulting in the vivid dreams characteristic of this stage. The brain's limbic system, which is responsible for emotions and motivation, is also active during REM sleep, contributing to the intense emotions and sensations experienced during dreams.

**Why Each Wrong Option is Incorrect**
**Option A:** REM sleep is necessary for physical growth and development. This is incorrect because while sleep is essential for physical growth and development, it is NREM sleep that is primarily responsible for these processes.

**Option B:** REM sleep is characterized by high muscle tone. This is incorrect because REM sleep is actually characterized by low muscle tone, also known as atonia, which is necessary to prevent acting out dreams.

**Option C:** REM sleep is the stage of sleep where the body's core temperature decreases. This is incorrect because it is actually NREM sleep that is associated with a decrease in core body temperature.

**Clinical Pearl / High-Yield Fact**
It's worth noting that sleep disorders, such as sleep apnea and insomnia, can disrupt REM sleep and lead to daytime fatigue and other symptoms. Additionally, certain medications, such as antidepressants, can also affect REM sleep and lead to changes in dreaming and other sleep-related behaviors.

**Correct Answer: B. REM sleep is characterized by low muscle tone.**