**Core Concept**
Early morning awakening is a characteristic symptom of depression, often seen in patients with a major depressive disorder. It is defined as waking up earlier than usual and being unable to fall back asleep, which can be distressing and interfere with daily functioning.

**Why the Correct Answer is Right**
The pathophysiology of early morning awakening in depression is complex and multifactorial. It is thought to be related to the disruption of the body's circadian rhythm and the abnormal regulation of the hypothalamic-pituitary-adrenal (HPA) axis. In depression, there is an increase in the release of cortisol and other stress hormones, which can lead to insomnia and early morning awakening. Additionally, the decreased activity of the neurotransmitter serotonin and the increased activity of the neurotransmitter norepinephrine can also contribute to the development of early morning awakening.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because early morning awakening is not typically associated with anxiety disorders, although anxiety can also disrupt sleep patterns.
**Option B:** This option is incorrect because insomnia is a sleep disorder characterized by difficulty falling asleep or staying asleep, but it is not the same as early morning awakening.
**Option C:** This option is incorrect because sleep apnea is a sleep disorder characterized by pauses in breathing during sleep, and it is not directly related to early morning awakening.

**Clinical Pearl / High-Yield Fact**
A key feature of early morning awakening in depression is its association with a "racing" or "ruminating" mind, making it difficult for patients to fall back asleep.

**Correct Answer:** C. Depression