Serotonin

Also known as 5-hydroxytryptamine, it is a neurotransmitter derived from the amino acid tryptophan and is involved in regulating mood, sleep, emotional behaviour and aggression.

Reference:

Sandyk, R. (1992). L-tryptophan in neuropsychiatry disorders: a review. International Journal of Neuroscience, 67(1-4), 127-144. https://doi.org/10.3109/00207459208994781