What role does melatonin play in the body?

Melatonin is primarily known for its crucial role in regulating biological rhythms, particularly the sleep-wake cycle. This hormone is produced by the pineal gland in response to darkness, signaling to the body that it is time to prepare for sleep. By influencing circadian rhythms—the 24-hour cycles that are integral to various physiological processes—melatonin helps to align our body's internal clock with the external environment. This regulation not only affects our sleep patterns but can also influence other biological functions that follow a diurnal cycle, such as hormone secretion and body temperature fluctuations.

The other roles mentioned in the options, such as regulating blood pressure or supporting the immune system, although they may have some relation to overall health and well-being, are not the primary functions attributed to melatonin. Melatonin's distinct influence on biological rhythms sets it apart as a key hormone for managing sleep and circadian alignment.