HCRT and Parkinson disease: For example, the orexin/hypocretin pathway, central to arousal stability, shows inconsistent stage-dependent alterations across α-synucleinopathies, with possible early compensatory increases followed by later reductions (a pattern emerging particularly in iRBD and PD), and may be better characterized with protocols that explicitly account for rest–activity phase and circadian rhythmicity (Raheel et al., 2024).