But wait, what does serotonin do again? As Aimee Harris-Newton, Psy.D., a double-board certified integrative and interventional psychologist previously told mbg, serotonin is a brain chemical messenger that relays messages between cells and receptors on other cells. Serotonin specifically is involved in feelings of happiness and contentment, and it plays a key role2 in sleep, mood, aggression, anxiety, pain, and sexual behavior.