CLOCK and Parkinson disease: Assessment of dim light melatonin onset (DLMO), a biomarker for the circadian clock phase, in PD patients has demonstrated that the phase angle of entrainment (the discrepancy between circadian phase and sleep/activity onset) is larger in PD patients on medication compared to PD un‐medicated patients, indicating a role for PD medication in alterations of melatonin rhythms and the relationship between the circadian and the sleep–wake cycles, in that dopaminergic therapy was associated with delayed sleep onset relative to the circadian phase as estimated by DLMO (Bolitho et al., 2014).