Which hormone is closely associated with the regulation of sleep-wake cycles?

Melatonin is the hormone that plays a crucial role in regulating sleep-wake cycles. It is produced by the pineal gland in response to darkness and helps signal to the body that it is time to sleep. Melatonin levels typically rise in the evening, promoting sleepiness, and decrease in the morning, contributing to wakefulness. This cyclical pattern is integral to maintaining the body's circadian rhythms, which govern various physiological processes, including sleep.

While cortisol, insulin, and serotonin have their own roles in sleep and wakefulness, they do not directly regulate the sleep-wake cycle to the same extent as melatonin does. Cortisol, for example, is more associated with stress response and increases alertness. Insulin primarily regulates blood glucose levels and metabolism, while serotonin is involved in mood regulation and can influence sleep, but it acts more as a precursor to melatonin rather than being the principal hormone directly affecting sleep-wake cycles. Thus, melatonin stands out as the key hormone specifically linked to the timing of sleep.