Melatonin is a hormone that is increasingly popular as a supplement to promote sound sleep, especially in people who travel between time zones or who work odd hours. Besides its function as synchronizer of the biological clock, melatonin is a powerful free-radical scavenger and wide-spectrum antioxidant. When compared to synthetic, antioxidants, melatonin proved to be a better protector against mitochondrial oxidative stress. Melatonin has been studied as a potential treatment of acid reflux disease, cancer, immune disorders, cardiovascular diseases, depression, seasonal affective disorder (SAD), circadian rhythm sleep disorders, sexual dysfunction, and insomnia in the elderly.