Mechanisms underlying circadian disruptions in PD remain unclear, but may involve afferent pathways to the SCN (e.g., impaired light transmission due to dopaminergic retinal degeneration) (Harnois and Di Paolo, 1990; Adam et al., 2013), the SCN itself (e.g., altered clock gene expression) (Hayashi et al., 2013; Mattam and Jagota, 2015), or downstream peripheral efferents (e.g., altered electrical output from the SCN) (Kudo et al., 2011; Videnovic and Golombek, 2013; Musiek, 2015). This evidence concerns the gene CLOCK and Parkinson disease.