The 24-hour (diurnal) rhythm of our body, such as the sleep-wake cycle, is regulated by the hormone: Calcitonin, Prolactin, Adrenaline, Melatonin?

Melatonin is a hormone secreted by the pineal gland in the brain. In animals, melatonin is involved in the regulation of the circadian rhythms of physiological functions including sleep timing, blood pressure regulation, seasonal reproduction, and many others. So, the correct answer is 'melatonin'.