CLOCK and Cluster headache: A study on an arctic cluster headache population found that cluster bouts more likely started around the equinoxes in March and September, times of the year when the daily change of natural light is most noticeable and the SCN increases in size.34,35 Intriguingly, a preliminary study from Portugal could find an increase in CLOCK gene expression in patients with cluster headache at the September equinox.36 These findings suggest an involvement and possible dysregulation of circadian rhythm in cluster headache producing specific diurnal and annual attack patterns.