Serotonin (5-hydroxytryptamine, or 5-HT) is a neurotransmitter synthesized in the central nervous system (CNS) as well as in enterochromaffin cells in the gastrointestinal tract of animals including humans.

Serotonin is also found in many foods and can also be made from its precursor, tryptophan.

In the central nervous system, serotonin plays an important role as a neurotransmitter in the modulation of anger, aggression, body temperature, mood, sleep, sexuality, appetite, and metabolism.

References

Crockett, Molly J.; Luke Clark, Golnaz Tabibnia, Matthew D. Lieberman, Trevor W. Robbins (2008-06-05). "Serotonin Modulates Behavioral Reactions to Unfairness". Science: 1155577. doi:10.1126/science.1155577.