Sohn JH, et al. - Considering the relation of cluster headache with seasonal and diurnal periodicity, which might be linked to sunlight and vitamin D metabolism, researchers investigated the serum vitamin D levels in these patients. From October 2016 to March 2018, they measured non-fasting serum 25(OH)D concentrations using a chemiluminescent immunoassay. It was common to find vitamin D deficiency (a concentration < 20 ng/mL) among these patients with cluster headaches, however, no clear conclusion regarding the role of vitamin D deficiency was drawn, except for its seasonal influence.