Melatonin is a hormone produced by the pineal gland in the brain that promotes sleep. It is produced cyclically in response to darkness and light. Spanish researchers, in a letter to the editor of the International Journal of Cardiology, speculated that melatonin protects the heart from chemicals called free radicals that promote heart stress. Supplementing the hormone might be a lifesaver during a heart attack when the heart is not getting enough oxygen. The research group is currently studying whether administering melatonin following a heart attack protects the heart muscle against free radical damage.
Source: International Journal of Cardiology, 182:54-55, 2015