The phrase "brain chemical serotonin" refers to a neurotransmitter, which is a chemical that carries signals between nerve cells in the brain. Serotonin is one of several neurotransmitters that play a role in regulating mood, appetite, sleep and other functions. It is often referred to as the "feel good" chemical because it contributes to feelings of well-being and happiness. Imbalances in serotonin levels have been linked to depression and anxiety disorders, among other conditions.