Alertness and behavioral performance depend on an animal's level of arousal. In vertebrates, reinforcement and maintenance of arousal in the cortex are ensured by diffuse inputs from neurons releasing biogenic amine neuromodulators. Fruit flies similarly use dopamine for arousal control, indicating an ancient evolutionary origin of this essential feature of the functioning brain.
Birman, S. (2005, July 12). Arousal mechanisms: Speedy flies don’t sleep at night. Current Biology. https://doi.org/10.1016/j.cub.2005.06.032