What hormone regulates the sleep-wake cycle?

• A. Cortisol
• B. Melatonin
• C. Serotonin
• D. Prolactin

Answer: (B)

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 to signal to the body that it is time to sleep. When the light decreases in the evening, melatonin levels increase, promoting sleepiness and helping to synchronize the circadian rhythm, which is the internal clock that governs various biological processes, including sleep.

The production of melatonin is closely tied to environmental light conditions, making it a critical component in managing the timing of sleep. As daylight approaches, melatonin production drops, promoting wakefulness. This natural rhythm is essential for maintaining healthy sleep patterns, influencing not just sleep duration but also sleep quality.

In contrast, other hormones mentioned, such as cortisol, serotonin, and prolactin, do play roles in various bodily functions, but they do not directly regulate the sleep-wake cycle in the same way that melatonin does. Cortisol, for instance, is involved in the stress response and typically peaks in the morning to promote alertness, while serotonin affects mood and can influence sleep indirectly but is not the primary regulator of sleep timing. Prolactin more commonly relates to reproductive and lactation functions rather than sleep regulation.