Tryptophan is considered an essential amino acid - suggesting it is essential for biological functioning but cannot be produced naturally by the body. Which is why tryptophan must be supplied through either diet or supplements.
Tryptophan is converted to 5-hydroxytryptophan (5-HTP) in the brain, which produces serotonin. Additionally - L-tryptophan aids in the production of melatonin, and kynurenine.
Tryptophan has been shown to assist with mood, depression, sleep, and even memory.