Which neurotransmitter regulates sleep, mood, attention, and learning, and too little of this neurotransmitter can cause depressed mood?

Answer: Serotonin and Dopamine

Dopamine is a neurotransmitter which is usually associated with attention and learning, and too of it little can be related to several forms of depression.

Also, Serotonin plays a part in the development of mood, appetite and is associated with sleep, whereas; too little of serotonin is related to depression.