Which hormone is key in regulating 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, signaling to the body that it is time to sleep. Increased levels of melatonin in the evening help to promote sleepiness, while lower levels during the day contribute to wakefulness. This natural hormone follows a circadian rhythm, closely aligning with the light-dark cycle, making it essential for maintaining a healthy sleep pattern.

In contrast, cortisol, which is often referred to as the stress hormone, tends to peak in the early morning to help promote alertness and energy, not sleep. Serotonin, although it plays several roles in mood regulation and is a precursor to melatonin, does not directly regulate sleep-wake cycles on its own. Adrenaline, or epinephrine, is associated with the body's 'fight or flight' response and is more involved in increasing alertness and arousal rather than facilitating sleep. Therefore, melatonin's specific role and timing in promoting sleep make it the key hormone in regulating sleep-wake cycles.