The two possible reasons are as follows: (1) Orexin is a polypeptide secreted by dorsal and lateral hypothalamic neurons, and it has been reported in some studies that the α-synuclein load in the hypothalamic area significantly increases in PD patients with sleep disorders [65], and that disruption of this area may lead to a decrease in orexin; (2) the activity of orexin neurons is regulated by DA [66] and other neurotransmitters (e.g., γ-GABA [67], and serotonin [68]), and disruption of these neurotransmitter systems in PD with sleep disorders leads to disturbances in orexin levels. This evidence concerns the gene HCRT and sleep disorder.