Which hormone is primarily involved in the regulation of the sleep-wake cycle?

Melatonin is the hormone primarily involved in the regulation of the sleep-wake cycle. It is produced by the pineal gland in response to darkness, signaling to the body that it is time to sleep. Melatonin's secretion is closely tied to the circadian rhythms, which dictate the body's internal clock, influencing sleep patterns and other physiological processes.

When light exposure decreases, the production of melatonin increases, promoting feelings of sleepiness and thereby helping to regulate the cycle of sleep and wakefulness. This makes melatonin critical for maintaining healthy sleep patterns, particularly in response to changes in light throughout the day.

The other hormones, while involved in various bodily functions, do not play as direct a role in regulating the sleep-wake cycle. Cortisol, for instance, is associated with stress and alertness and typically peaks in the early morning to help promote wakefulness. Serotonin, although it is related to mood regulation and can influence sleep indirectly, does not primarily govern the sleep-wake cycle like melatonin does. Dopamine is primarily involved in reward, motivation, and motor control, and is not a direct regulator of sleep patterns either.