Neurotransmitter Functions in Mood Regulation

The human nervous system is a complex network responsible for processing and transmitting signals throughout the body. Neurotransmitters play a crucial role in this network by allowing communication between neurons. One specific neurotransmitter is known for its function in mood regulation and its significant role in modulating both the sleep-wake cycle and arousal response. In certain diseases, such as depression, the dysregulation of this neurotransmitter can lead to profound psychological effects. Which neurotransmitter is primarily associated with these functions?