Tryptophan is an amino acid necessary for normal growth in infants and for nitrogen balance in adults. It is an essential amino acid, which means your body cannot produce it -- you must get it from your diet. Tryptophan can be found in turkey, chicken, milk, cheese, fish, eggs, tofu, soy, sesame and pumpkin seeds, nuts, peanuts, and peanut butter. It is used by your body to help make niacin and serotonin, which is thought to produce healthy sleep and a stable mood.