CLOCK and Cluster headache: The rs1801260 polymorphism of the CLOCK gene has been reported to modify diurnal preference (Katzenberg et al., 1998), however, so far no association has been identified between cluster headache and this SNP (Rainero et al., 2005; Cevoli et al., 2008; Zarrilli et al., 2015; Fan et al., 2018).