Which of the following hormones regulates the sleep-wake cycle?

Melatonin is the hormone that primarily regulates the sleep-wake cycle. It is produced by the pineal gland in response to darkness, signaling to the body that it is time to prepare for sleep. Melatonin levels generally rise in the evening, helping to induce sleepiness, and fall in the morning, promoting wakefulness. This natural rhythm is crucial for maintaining a healthy sleep cycle and is influenced by external light conditions.

Other hormones mentioned, such as insulin, aldosterone, and thymosin, play distinct roles in the body, but they do not have a direct function in regulating sleep patterns. Insulin is involved in glucose metabolism, aldosterone regulates sodium and potassium levels and fluid balance, and thymosin is related to immune system function, specifically in T cell development. Therefore, the correct identification of melatonin highlights its specific role in managing circadian rhythms and sleep.