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