Serotonin is a chemical that acts as a neurotransmitter (messenger in the brain) and a hormone in the body, playing a vital role in regulating mood, sleep, digestion, blood clotting, and other bodily functions. Made from the amino acid tryptophan, serotonin helps transmit signals between cells in the brain and other organs. Low serotonin levels can lead to depression, while excessively high levels can be harmful.