From a dietary amino acid to a night-time hormone
Melatonin is the end product of a multi-step biosynthetic pathway that begins with the essential amino acid tryptophan obtained from food. Tryptophan is first hydroxylated to 5-hydroxytryptophan (5-HTP), then decarboxylated to serotonin. In the pineal gland at night, serotonin is converted by the enzyme AANAT into N-acetylserotonin and finally methylated to melatonin. AANAT activity is the key rate-limiting, light-sensitive step, which is why melatonin output tracks darkness.
Because serotonin sits midway in this chain, the same precursor pool feeds both daytime serotonin signaling and night-time melatonin production.
Why eating tryptophan does not flood the brain with melatonin
Tryptophan competes with other large amino acids to cross into the brain, and the conversion steps are tightly regulated, so simply eating more protein does not reliably raise melatonin. This is why the pathway is better understood as a regulated pipeline than a dose-in, dose-out system, and it tempers claims that specific foods sharply boost melatonin.
This is general educational information, not medical advice. Supplementing precursors such as 5-HTP can interact with serotonergic medication and should be discussed with a qualified professional.