Source Naturals L-Tryptophan 500 mg. Capsules promote mood, relaxation, and sleep. The essential amino acid Source Naturals L-Tryptophan 500 mg. Capsules helps support relaxation, restful sleep, and feeling better. Source Naturals L-Tryptophan 500 mg. Capsules play a part in the synthesis of both melatonin and serotonin, hormones involved with mood and stress response. Source Naturals L-Tryptophan 500 mg. Capsules also supports immune functions because it is the body's precursor to the kynurenines that regulate immunity. If needed, Source Naturals L-Tryptophan 500 mg.