It's inferred that the majority of melatonin is created inside the mitochondria, independent of the pineal daylight associated melatonin. Melatonin has a 3 billion year history of use inside cells as a highly efficient anti-oxidant. It functions as an anti-oxidant for both plants and animals. It stimulates synthesis of other antioxidants like glutathione. It's the "swiss army knife" of anti-oxidants and uniquely positioned as the "fox in the hen house" inside the mitochondria where many free radicals originate. Melatonin reverses the Warburg effect and aids in arresting cancer cell growth.