L-Tryptophan is an essential amino acid important in human nutrition for the synthesis of melatonin and serotonin, hormones regulating mood and stress response. L-Tryptophan helps support relaxation, sleep, positive mood and immune function.* As an essential amino acid, it is not synthesized by the body and must be obtained fromt he diet. NOW&ampreg; L-Tryptophan is pharmaceutically pure - every lot is tested to be free of Peak E and microbial contamination.