**Core Concept:** Hormones play a crucial role in regulating physiological processes and maintaining homeostasis in the body. During sleep, certain hormones are secreted to ensure proper sleep regulation and overall health.

**Why the Correct Answer is Right:** Melatonin is the hormone that is increased during sleep. Melatonin is a hormone produced by the pineal gland in the brain and is involved in regulating sleep-wake cycles. It is synthesized from the amino acid tryptophan and its secretion peaks at night, particularly in the absence of light. This helps maintain the circadian rhythm and establish a natural sleep-wake cycle.

**Why Each Wrong Option is Incorrect:**

A. Cortisol is a hormone known for its role in the body's response to stress and is secreted by the adrenal glands, not related to sleep.

B. Insulin is a hormone responsible for regulating blood sugar levels, not associated with sleep regulation.

C. Growth hormone is involved in promoting growth and tissue repair, not directly related to sleep.

D. Growth hormone is regulated by the sleep-wake cycle, but its secretion is not directly correlated with sleep itself.

**Clinical Pearl / High-Yield Fact:** Melatonin plays a significant role in maintaining circadian rhythm and promoting sleep, ensuring a restful night's sleep. Understanding the hormonal regulation of sleep can help in diagnosing and treating sleep disorders and maintaining overall health.