Serotonin is a neurotransmitter, on the other hand, Melatonin is a neurotransmitter-like substance. Serotonin produces signals between nerve cells for responses. Melatonin supervises our body clock. Darkness helps produce Melatonin, and light helps increase Serotonin. Serotonin and Melatonin do opposite jobs, and they should work in harmony to keep the body balanced.