I am a neurotransmitter mainly involved in mood, appetite, sleep, and impulsivity. A large body of research says that I am strongly implicated in depression. What is my name?

Answer: Serotonin

Explanation:

Serotonin is an hormone, derived from the amino acid, tryptothan. It is involved in vasodilation, sleep regulation, mood changes, capillary permeability, and causes contraction of smooth muscles like that of the stomach to signal hunger.