Advanced Nutrition by Zahler Purified L-Tryptophan High Potency Serotonin Enhancer Mind & Spirit Dietary Supplement Feel Better, Everyday Free of Gluten Kosher Pareve Dairy Free Soy Free High Potency Serotonin Enhancer L-Tryptophan is used by the brain as a precursor to serotonin and melatonin, neurotransmitters that are necessary for normal sleep. Adequate sleep is crucial for a healthy immune and nervous system. In addition, changes in the level of serotonin in the brain can impact mood and satiety. Zahler L-Tryptophan is a potent, purified form, which effectively crosses the blood brain barrier.