VDR and sleep disorder: Prono and colleagues [23] investigated the association between vitamin D levels and sleep disorders, including OSA, supposing that low levels of the vitamin could play a role in the pathogenesis of these disorders; although the exact mechanism by which the vitamin D level impacts sleep regulation is still unclear, the presence of vitamin D receptors (VDRs) in areas of the brainstem, which are involved in sleep regulation, could explain the reason why a decrease of this vitamin in the blood induces sleep alterations.