PER1, PER2, and PER3 genes which are considered to be light sensitive clock genes (Sosniyenko et al., 2009) have also been investigated in relation to cluster headache in a Swedish case-control study of 524 cluster headache cases and 680 controls (Jennysdotter Olofsgård et al., 2021). The gene discussed is PER3; the disease is Cluster headache.