One of the essential amino acids. It is a heterocyclic compound that is found in small amounts in most proteins. It plays an important role in the growth and development of infants and in the biosynthesis of serotonin and niacin (thus, deficiency of both niacin and tryptophan causes pellagra). Its occurrence in milk is the reason milk helps people sleep. It is used in medicine and nutrition research, in enriched foods, and as a dietary supplement.