In this study, we measured orexin levels in the CSF samples obtained from patients with PD to clarify the role of orexin as a biomarker of PD, in the following particulars: (i) its difference between PD and other diseases, (ii) its association with the severity of sleep disturbances and other clinical characteristics, and (iii) its relationship with and predictive value of the longitudinal course of PD. Here, HCRT is linked to Parkinson disease.