Which hormone is associated with regulating daily patterns of sleep and wakefulness?

Melatonin is the hormone primarily involved in regulating daily patterns of sleep and wakefulness. It is produced by the pineal gland in response to darkness, helping to signal the body that it is time to prepare for sleep. This hormone plays a crucial role in establishing the circadian rhythm, which is the body's internal clock that orchestrates various physiological processes, including the sleep-wake cycle.

As light levels decrease in the evening, melatonin secretion increases, promoting feelings of drowsiness and facilitating the onset of sleep. Conversely, when light levels rise in the morning, melatonin production diminishes, helping the body to wake up and become alert.

The other hormones listed are not primarily responsible for regulating sleep patterns. Cortisol, for instance, is more associated with the body's response to stress and is often elevated in the morning, helping to promote wakefulness. Serotonin, while it does play a role in mood regulation and can influence sleep, is not the main hormone that regulates sleep-wake cycles. Growth hormone, on the other hand, is involved in growth and metabolic processes, and is released during sleep but does not directly regulate sleep patterns in the way melatonin does. Thus, melatonin is the correct answer due to its specific role in