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. Created almost exclusively in the Dorsal Raphe Nuclei (DRN). Contrast with endorphines and dopamine.

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