**Core Concept**
Growth hormone (GH) secretion is a complex process regulated by the hypothalamus and the pituitary gland. GH secretion is influenced by various factors, including sleep, exercise, nutrition, and stress, which ultimately affect the release of GH-releasing hormone (GHRH) and somatostatin.

**Why the Correct Answer is Right**
GH secretion is primarily regulated by the balance between GHRH and somatostatin. Somatostatin is a potent inhibitor of GH release, and its increased secretion can decrease GH levels. This is particularly relevant during sleep, when somatostatin levels increase, causing a decline in GH secretion. Additionally, factors such as stress, which can stimulate the release of cortisol, can also inhibit GH secretion.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because dopamine, a neurotransmitter, can actually stimulate GH release.
**Option B:** Incorrect because growth hormone-releasing hormone (GHRH) is a potent stimulator of GH release, not a decrease.
**Option C:** Incorrect because insulin-like growth factor-1 (IGF-1) is a downstream effector of GH, not a regulator of GH secretion.

**Clinical Pearl / High-Yield Fact**
GH secretion is pulsatile and occurs primarily during deep sleep, with each pulse lasting around 2-3 hours. Understanding the regulation of GH secretion is crucial for diagnosing and managing conditions such as growth hormone deficiency.

**Correct Answer: D. Somatostatin**