This article discusses the dual sources of melatonin in mammals, including humans, and their different functions. The primary source of melatonin, referred to as Source #1, is the pineal gland, which produces melatonin in a circadian rhythm, with peak synthesis and release occurring at night. However, less than 5% of the total melatonin produced in mammals is derived from the pineal gland. The melatonin rhythm primarily influences the circadian clock at the suprachiasmatic nucleus (SCN) and peripheral organs via receptor-mediated actions. The second source of melatonin, Source #2, is produced in multiple tissues throughout the body, likely in the mitochondria of these cells, and is concerned with metabolic regulation. This extrapineal melatonin does not exhibit a circadian rhythm and acts locally in its cell of origin, possibly through paracrine mechanisms. The concentration of melatonin in these cells is proposed to be determined by the subcellular redox state, and its synthesis may be inducible under stressful conditions. The review emphasizes the role of extrapineal melatonin in determining redox homeostasis and other critical metabolic effects. The evolution of melatonin is discussed, suggesting that it first appeared in prokaryotes and was later incorporated into eukaryotic cells through endosymbiosis, evolving into mitochondria. Melatonin's antioxidant properties are highlighted, as it is highly effective in neutralizing reactive oxygen species (ROS) and reactive nitrogen species (RNS), protecting cells from oxidative damage. The article also discusses the interaction between Source #1 and Source #2 melatonin, particularly in the immune-pineal axis, where nocturnal melatonin surge is suppressed by proinflammatory cytokines while inducing melatonin synthesis in immune cells. Overall, both sources of melatonin are critical for health maintenance, and their roles in various physiological processes are extensively reviewed.This article discusses the dual sources of melatonin in mammals, including humans, and their different functions. The primary source of melatonin, referred to as Source #1, is the pineal gland, which produces melatonin in a circadian rhythm, with peak synthesis and release occurring at night. However, less than 5% of the total melatonin produced in mammals is derived from the pineal gland. The melatonin rhythm primarily influences the circadian clock at the suprachiasmatic nucleus (SCN) and peripheral organs via receptor-mediated actions. The second source of melatonin, Source #2, is produced in multiple tissues throughout the body, likely in the mitochondria of these cells, and is concerned with metabolic regulation. This extrapineal melatonin does not exhibit a circadian rhythm and acts locally in its cell of origin, possibly through paracrine mechanisms. The concentration of melatonin in these cells is proposed to be determined by the subcellular redox state, and its synthesis may be inducible under stressful conditions. The review emphasizes the role of extrapineal melatonin in determining redox homeostasis and other critical metabolic effects. The evolution of melatonin is discussed, suggesting that it first appeared in prokaryotes and was later incorporated into eukaryotic cells through endosymbiosis, evolving into mitochondria. Melatonin's antioxidant properties are highlighted, as it is highly effective in neutralizing reactive oxygen species (ROS) and reactive nitrogen species (RNS), protecting cells from oxidative damage. The article also discusses the interaction between Source #1 and Source #2 melatonin, particularly in the immune-pineal axis, where nocturnal melatonin surge is suppressed by proinflammatory cytokines while inducing melatonin synthesis in immune cells. Overall, both sources of melatonin are critical for health maintenance, and their roles in various physiological processes are extensively reviewed.