**Core Concept**
Major Depression, also known as Unipolar Depression, is a mood disorder characterized by persistent feelings of sadness, hopelessness, and a loss of interest in activities. It is a complex condition that affects multiple neurotransmitter systems, including serotonin, dopamine, and norepinephrine.

**Why the Correct Answer is Right**
Major Depression is often associated with decreased levels of serotonin and norepinephrine, which play a crucial role in mood regulation. The condition is also linked to abnormalities in the hypothalamic-pituitary-adrenal (HPA) axis, leading to increased cortisol levels. The pathophysiology of Major Depression involves disruptions in these neurotransmitter systems and the HPA axis, contributing to the development of depressive symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:**
* Decreased cortisol levels are not typically seen in Major Depression; rather, increased cortisol levels are observed due to HPA axis dysregulation.

**Option B:**
* Although sleep disturbances are common in Major Depression, decreased REM sleep is not a characteristic feature of the condition.

**Option C:**
* Increased REM sleep, not decreased, is often seen in Major Depression, and is associated with the development of depressive symptoms.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Major Depression is a heterogeneous condition, and its pathophysiology can vary significantly between individuals. A comprehensive diagnostic evaluation, including a thorough medical and psychiatric history, is crucial for accurate diagnosis and treatment.

**Correct Answer: D**