Trial Size. MOOD RELAXATION SLEEP. The essential amino acid L-tryptophan helps support relaxation restful sleep and feeling better. It plays a part in the synthesis of both melatonin and serotonin hormones involved with mood and stress response. L-Tryptophan also supports immune functions because it is the body’s precursor to the kynurenines that regulate immunity. If needed L-tryptophan converts to niacin in the body which supports circulation a healthy nervous system the metabolism of food and the production of hydrochloric acid for the digestive system. Source Naturals L-TRYPTOPHAN is extremely pure and is regularly tested to ensure the highest standards of quality.