Which neurotransmitter is primarily associated with the regulation of hunger, sleep, arousal, and mood?

Serotonin plays a crucial role in various physiological and psychological processes, including the regulation of hunger, sleep, arousal, and mood. It is a neurotransmitter that significantly impacts mood regulation and emotional well-being; low levels of serotonin are often linked to mood disorders such as depression. Additionally, serotonin influences appetite control, contributing to the sensation of satiety after eating. Its role in regulating the sleep-wake cycle is also notable, as it affects sleep patterns and can promote wakefulness or relaxation depending on its levels.

This multifaceted function makes serotonin an essential neurotransmitter in maintaining homeostasis in behaviors related to hunger, sleep, and mood, which aligns with the criteria set out in the question.