Melatonin is released from the pineal gland in the brain. Darkness stimulates its release. Melatonin plays a crucial role in getting restful, restorative sleep. Melatonin also regulates our circadian rhythm, immune system, and may be important in the prevention of certain cancers (breast and prostate).
Most people have declines in melatonin release after age 40. Melatonin is also a very potent antioxidant, neutralizing free radicals before they can cause permanent damage to our mitochondria and DNA.