Protocol: Investigators recruited eligible Han Chinese participants (aged 25-39) if they had smoking at least 10 cigarettes per day for at least 1 year. Participants excluded if they had undergone cardiovascular disease, or systolic blood pressure above 140 mm Hg, or diastolic blood pressure above 90 mmHg, or psychiatric disorders, or cancer, or pregnant, or lactating, or taking antipsychotic drugs orally during the 2 weeks of the trial. They were randomly divided participants into non-smoking with oral placebo, non-smoking with oral melatonin, smoking with oral placebo, and smoking with oral melatonin. They are oral melatonin 3 mg/day or placebo for 2 weeks. Blood samples (about 3 milliliter) were taken at baseline and after 2 weeks of treatment. Through a series hospital clinical laboratory and related ELISA kits to detect endothelial cell injury in serum markers platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31)， intercellular adhesion molecule-1 (ICAM - 1), vascular cell adhesion molecule-1 (VCAM 1), endothelin-1 (ET-1), endothelial nitric oxide synthase (eNOS), nuclear erythroid 2-related factor 2 (Nrf - 2), NAD(P)H quinone oxidoreductase-1 (NQO-1), catalytic glutamate cysteine ligase (GCLC), heme oxygenase-1 (HO-1), free fatty acid (FFA), total cholesterol (TC), triglyceride (TG), low density lipoprotein (LDL), hypersensitive c-reactive protein (hsCRP), fibrinogen (Fbg), and free fatty acids (FFA), through the above vascular endothelial damage index analysis whether melatonin have protective effect against smoke-induced vascular injury. All participants and study investigators were unaware of treatment allocation throughout the trial. This trial is approved by the Ethical Committee of Peking Union Medical College Hospital (No JS-863). All participants completed a questionnaire and signed an informed consent document. Otherwise, they will get appropriate economic compensation. To achieve treatment concealment, melatonin and placebo in appearance and package were identically. Trial associates monitored compliance with the masking procedure throughout the trial. All participants and study investigators were unaware of treatment allocation throughout the study. The randomization codes remained sealed until after data collection and cleaning, and completion of a masked analysis. The study team monitored and classified protocol deviations. Investigators summarized baseline clinical and demographic characteristics with descriptive statistics and then determined by the Univariate Analysis of Variance. All the data analyses were done using statistical software SPSS 20.0.