Melatonin, 5-methoxy-N-acetyltryptamine, is a hormone found in all living creatures from algae to humans, at levels that vary in a daily cycle.Many biological effects of melatonin are produced through activation of melatonin receptors,while others are due to its role as a pervasive and extremely powerful antioxidant with a particular role in the protection of nuclear and mitochondrial DNA.Melatonin is secreted by the pineal gland in the brain and is important in the regulation of many hormones in the body. Among its key roles, melatonin controls the body's circadian rhythm, an internal 24-hour time-keeping system that plays an important role in when we fall asleep and when we wake up. Darkness stimulates the release of melatonin and light suppresses its activity. Normal melatonin cycles are disrupted when we are exposed to excessive light in the evening or too little light during the daytime. For example, jet lag, shift work, and poor vision can disrupt melatonin cycles.