**Core Concept**
The hypothalamic-pituitary-adrenal (HPA) axis plays a crucial role in the body's response to stress, with adrenocorticotropic hormone (ACTH) being a key regulator. ACTH stimulates the adrenal glands to produce cortisol, a vital hormone for responding to stress and maintaining homeostasis.

**Why the Correct Answer is Right**
During times of stress, the hypothalamus releases corticotropin-releasing hormone (CRH), which stimulates the pituitary gland to release ACTH. ACTH then triggers the adrenal glands to produce cortisol, leading to an increase in cortisol levels. This process is a vital response to stress, and the peak levels of ACTH are observed during this process. The correct answer reflects the physiological response to stress, which is a critical concept in understanding the HPA axis.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately reflect the physiological response to stress. While cortisol levels may fluctuate throughout the day, ACTH levels are highest during periods of stress.
* **Option B:** This option is incorrect because it is the opposite of the correct answer. ACTH levels are not lowest during periods of stress, but rather highest.
* **Option C:** This option is incorrect because it is not a specific time point when ACTH levels are highest. ACTH levels can fluctuate throughout the day, but the correct answer reflects the peak levels during stress.

**Clinical Pearl / High-Yield Fact**
The HPA axis is a critical regulator of the body's response to stress, and understanding the physiological response to stress is essential for managing patients with adrenal disorders or those undergoing stress-related medical conditions.

**Correct Answer:** C. During periods of stress, such as after waking or in response to a stressful event.