Answer:
b. circadian rhythm
Explanation:
Circadian rhythm refers to an internal rhythm that operates during the 24-hour day. It represents the patterns of biological activity such as the sleep-wake cycle that occurs on a cycle of about 24 hours. The visual inputs from the retina to the suprachiasmatic nucleus of the hypothalamus regulate the release of melatonin. Reduced secretion of melatonin during day and increased levels during night regulate the rhythmic changes in sleep, wakefulness and body temperature. For example, as night arrives, the increased melatonin secretion stimulates sleep and lowers down the body temperature.