In mammals, melatonin is biosynthesized in the pineal gland from the essential dietary amino acid tryptophan, with serotonin produced as an intermediate. In general Melatonin acts as an endocrine hormone.
Many biological effects of melatonin are generated through activation of melatonin receptors, while others are due to its role as a powerful antioxidant, being a direct scavenger of OH, O2−, and NO.
The serum and saliva melatonin levels in humans show a marked circadian rhythm characterized by very low levels during day time and high levels during night time.
In humans, 90% of melatonin is cleared in a single passage through the liver, a small amount is excreted in urine, and a small amount is found in saliva.