What hormone regulates the sleep-wake cycle and is produced by the pineal gland?

Melatonin is the hormone that plays a crucial role in regulating the sleep-wake cycle, and it is produced by the pineal gland. This gland is located in the brain and is sensitive to light exposure; it synthesizes melatonin primarily during the night. The secretion of melatonin increases in response to darkness, which helps signal to the body that it is time to prepare for sleep, thereby contributing to the regulation of circadian rhythms.

Melatonin levels decrease in the presence of light, particularly daylight, which helps to facilitate wakefulness. This cycle is important for maintaining healthy sleep patterns, and disruptions in melatonin production can lead to sleep disorders.

In contrast, the other hormones listed have different roles in the body. Cortisol is related to the stress response and peaks in the early morning to promote alertness. Serotonin is a neurotransmitter that contributes to feelings of well-being and happiness and serves as a precursor to melatonin but does not directly regulate the sleep-wake cycle. Adrenaline, also known as epinephrine, is involved in the body’s "fight or flight" response and increases heart rate and energy levels. Thus, melatonin stands out as the primary regulator of the sleep-wake cycle.