The aim of current study was to evaluate the therapeutic effects of melatonin on sleep quality in SM-exposed patients with sleep disorders. This randomized, double-blind, placebo controlled clinical trial was conducted in chemical injury research center of Baqiatallah University of Medical Sciences. According to the inclusion and exclusion criteria, as well as basic clinical findings and spirometric data, 30 SM-injured patients with mild (FEV1≥80%) and moderate (50% ≤ FEV1≤ 80% ) lung injuries which their sleep disorders were confirmed using polysomnography at sleep clinic of Baqiatallah hospital were included in the current survey. Patients who had a history of cigarette smoking or exposure to any other respiratory pollutants, rhinitis allergique or other allergic diseases before exposure to sulfur mustard, history of asthma, lung cancer and pulmonary tuberculosis, acute infection at upper and lower respiratory system, history of drugs consumption that are associated with pulmonary complications, and history of systemic diseases or other chronic abnormalities which are associated with lung problems (such as heart disorders, kidney diseases, hepatitis, cirrhosis), were excluded from the study. Initially, quality of sleep and daytime sleepiness and the risk of obstructive sleep apnea were assessed by the Pittsburgh Sleep Quality Index (PSQI), the Epworth Sleepiness Scale (ESS) and STOP-Bang Questionnaires, respectively then patients were randomized to the melatonin or placebo group and Melatonin ( Nature Made ) or placebo (Tehran-chemistry Company) were supplied in identical 3- mg tablets to be taken in a single dose 1 hour before bedtime for 8 weeks. Assessment of the sleep quality, daily sleepiness and the risk of obstructive sleep apnea were repeated at the end of the treatment for comparison.