Name the hormone responsible for the regulation of sleep-wake cycles.

The hormone responsible for the regulation of sleep-wake cycles is melatonin. Melatonin is produced by the pineal gland in response to darkness and plays a crucial role in signaling the body that it is time to sleep. Its levels typically rise in the evening, peak during the night, and decrease in the morning as light conditions change. This rhythmic production helps to regulate circadian rhythms, the natural biological processes that cycle roughly every 24 hours, facilitating the regulation of sleep patterns.

In contrast, cortisol is a hormone associated with stress response and is typically elevated in the early morning to promote wakefulness. Serotonin is primarily a neurotransmitter involved in mood regulation, but it also plays a role in sleep since it is a precursor to melatonin. Growth hormone is primarily involved in growth and metabolism and does not have a direct regulatory effect on sleep-wake cycles.