Which hormone regulates the sleep-wake cycle?

• A. Cortisol
• B. Testosterone
• C. Melatonin
• D. Insulin

Answer: (C)

The hormone responsible for regulating the sleep-wake cycle is melatonin. Melatonin is produced by the pineal gland in response to darkness, signaling to the body that it is time to prepare for sleep. Its levels typically rise in the evening, peak during the night, and decrease in the morning as light exposure increases, helping to establish and maintain the circadian rhythm.

In contrast, cortisol, testosterone, and insulin do not primarily influence sleep patterns in the same direct manner as melatonin. Cortisol is more involved in the body's stress response and peaks in the early morning to help promote wakefulness. Testosterone plays roles in various bodily functions, including the development of male characteristics and reproductive processes, but does not directly regulate sleep cycles. Insulin is primarily involved in glucose metabolism and does not have a significant role in sleep regulation. Thus, melatonin is the key hormone in managing the biological processes that dictate the sleep-wake cycle.