The discovery of antioxidative properties of melatonin is consistent with an ancient role of this molecule in contributing to survival under high oxygen levels or exposure to UVR.
Because of melatonin production in mitochondria scavenging of free oxygen radicals has likely been a primary role of melatonin in evolution originating around 2.5–3 billion years ago
In all organisms, melatonin is formed from tryptophan via serotonin.
In vertebrates, melatonin is mainly perceived as the hormone of the pineal gland.
The mammalian pineal gland represents the major source of melatonin in the blood, & in the cerebrospinal fluid of the third ventricle of the brain, where it contributes to the regulation of the circadian system
Melatonin controls pigmentation changes by aggregation of MELANIN into the melanocytes within the skin, causing the skin to change color.
This interaction is also responsible for the paler skin color of elderly people and those with insomnia.