HCRT and Parkinson disease: As a result of the high prevalence of sleep disturbances in patients with PD, there is a growing interest in orexin as fluctuations in CSF orexin and degeneration of orexinergic neurons have been associated with the development of sleep disorders that mimic early stages of alpha-synucleinopathy in other groups of patients (e.g. secondary RBD in narcolepsy) [29, 30, 57, 61–65, 95].