Which hormone is primarily responsible for regulating the sleep-wake cycle?

Melatonin is the hormone primarily responsible for regulating the sleep-wake cycle. It is produced by the pineal gland in response to darkness and helps signal the body that it is time to sleep. The secretion of melatonin typically increases in the evening as light diminishes, promoting feelings of sleepiness, and decreases in the morning when light is present, which helps to wake the body up.

In addition to melatonin's role in sleep, it is also involved in regulating circadian rhythms, which are the physical, mental, and behavioral changes that follow a daily cycle. These rhythms are primarily influenced by light and darkness in the environment.

The other hormones mentioned, although they play important roles in various bodily functions, do not primarily regulate the sleep-wake cycle. Serotonin is predominantly known as a neurotransmitter involved in mood regulation and can indirectly affect sleep through its conversion to melatonin. Cortisol is a stress hormone that rises in the morning to promote wakefulness and is involved in the body’s response to stress, while insulin manages blood sugar levels. Thus, when it comes to the regulation of the sleep-wake cycle, melatonin stands out as the key hormone.