DRD2 and insomnia: DA, acting through both D1 and D2 receptors, exerts a stimulatory effect on the excitability of the HPA axis induced by stress, and D1R and D2R antagonists can reduce the activity of the HPA axis under stress [28], which suggests that dopamine and its receptors in the HPA axis may be involved in the regulation of sleep-wake cycles in insomnia.