Tryptophan is an essential amino acid critical for protein synthesis in humans and animals. It serves as the biochemical precursor to serotonin (5-hydroxytryptamine), melatonin, and niacin (vitamin B3) via the kynurenine pathway. The compound features an indole ring system attached to an alanine side chain, enabling its role in neurotransmitter regulation, mood stabilization, sleep-wake cycles, and immune modulation. Dietary sources include turkey, eggs, cheese, and soy products. It is metabolized primarily in the liver and brain, with excess leading to urinary excretion of metabolites. Tryptophan deficiency can contribute to pellagra symptoms when combined with niacin shortage. It is commonly used in dietary supplements for sleep and mood support and is produced industrially via fermentation.