What is a neurotransmitter that has been linked to sleep eating mood pain and depression?

Serotonin (5-hydroxytryptamine, or 5-HT) is the neurotransmitter associated with reducing anxiety, reducing aggression, reducing appetite, increasing somnolence (sedation and quality of sleep), and reducing depression.

A deficiency of this neurotransmitter, or receptor mutations leading to decreased binding affinity of one's own neurotransmitter levels can lead to any of inverse symptoms listed above (depression, anxiety, aggression, increased hunger or overeating, sleep disorders, etc.).