Circannual worsening in cluster headache correlates with daylight change

Plain Language Summary Cluster headache attacks typically occur with circadian rhythmicity and seasonal variability.In this article, we show that circannual rhythmicity of cluster headache attacks with its peak occurrence in spring and autumn could be explained by the rapidity in daylight alterations over the photoperiod, which is not constant but maximal at the time of equinoxes in spring. The peak of cluster attack occurrence at the equinoxes in spring and autumn coincides with greater leaps in day length, leading to interfere with synchronization of endogenous pacemaker systems. Interactions with the serotonergic system, also following circannual fluctuations are discussed.