Serotonin is an important chemical and neurotransmitter in the human body, believed to help regulate mood and social behavior, appetite and digestion, sleep, memory, and sexual desire and function. As a chemical nerve cells produce, it sends signals between your nerve cells. Serotonin is found mostly in the digestive system, although it’s also in blood platelets and throughout the central nervous system. Serotonin is made from the essential amino acid tryptophan.