What function does melatonin primarily have in the body?

Melatonin primarily regulates the sleep-wake cycle, signaling the body when it is time to sleep. It is a hormone produced by the pineal gland in response to darkness, helping to facilitate sleep by inducing feelings of drowsiness and lowering body temperature. This aligns with its role in managing circadian rhythms, which are the natural processes that regulate the sleep-wake cycle in alignment with the day-night cycle.

The other options, while relevant to various bodily functions, do not accurately represent the primary role of melatonin. For instance, the regulation of body temperature and alertness involves different systems and hormones, such as those related to thermoregulation and the adrenal response. Similarly, metabolism is influenced by various hormones like insulin and thyroid hormones rather than melatonin itself. Therefore, the role of melatonin in inducing sleep stands out as its defining function.