Screening, Part 2: In positive case, the patients will be asked to sign an informed consent. Beck Depression Inventory (BDI), Mini International Neuropsychiatric Interview (M.I.N.I.) and Insomnia Severity Index (ISI) will be administered to exclude depression, sleep disorders and major psychiatric disorders respectively. Thereafter blood will be tested according to table 1. Patients with vitamin D levels below 20 µg/L will undergo randomization. Participants will be informed about their results of vitamin D levels by the telephone, a treatment appointment within 2 weeks after screening will be arranged for participants with vitamin D levels ≤ 20 µg/L.