**Core Concept**
Sleep is a complex physiological process characterized by distinct stages, which are essential for physical and mental restoration. The different stages of sleep are defined by specific patterns of brain waves, muscle tone, and other physiological changes. The two main types of sleep are non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep.

**Why the Correct Answer is Right**
The stages of sleep are primarily defined by patterns in brain waves, specifically the presence of alpha, theta, and delta waves, as well as the occurrence of rapid eye movements and muscle atonia. The stages of sleep include stage 1 NREM sleep, stage 2 NREM sleep, stage 3 NREM sleep (also known as slow-wave sleep), and REM sleep. These stages are characterized by distinct changes in brain waves, heart rate, and other physiological parameters.

**Why Each Wrong Option is Incorrect**
**Option A:** is incorrect because sleep stages are not defined by heart rate variability. While heart rate does decrease during sleep, it is not a reliable indicator of the different stages of sleep.

**Option B:** is incorrect because sleep stages are not defined by core body temperature. Core body temperature does decrease during sleep, but this is not a specific characteristic of the different stages of sleep.

**Option C:** is incorrect because sleep stages are not defined by respiratory rate. Respiratory rate may change during sleep, but this is not a reliable indicator of the different stages of sleep.

**Option D:** is incorrect because sleep stages are not defined by blood pressure. Blood pressure may change during sleep, but this is not a specific characteristic of the different stages of sleep.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that sleep deprivation can have significant consequences on physical and mental health, including impaired cognitive function, mood disorders, and increased risk of chronic diseases like diabetes and cardiovascular disease.

**Correct Answer: D. Blood pressure.**