 After written informed consent, patients will be randomized to receive a single dose of oral vitamin D 400 000 UI or placebo. Subjects will undergo medical screening before randomisation into placebo or active group. Follow-up measurements will be performed at the beginning day 0 and after 32 days of vitamin D supplementation: clinical examination, biological check up and sleep parameter evaluation. Patients will be instructed to stop using continuous positive airway pressure device for two nights before nocturnal sleep studies. 