Which neurotransmitter affects mood, hunger, sleep, and arousal; undersupply linked to depression?

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Serotonin helps regulate mood, appetite, sleep, and arousal across multiple brain systems. When serotonin signaling is low, it can contribute to depressive symptoms, which is why an undersupply is linked to depression. This neurotransmitter influences mood directly and also signals fullness and energy levels that affect hunger and sleep-wake cycles, making it the best fit for all four functions. Other neurotransmitters play related roles (norepinephrine with alertness, dopamine with reward, acetylcholine with learning), but serotonin uniquely ties together mood, eating, and sleep in a single system. Treatments that boost serotonin availability often improve mood and help regulate sleep and appetite, highlighting its central role.