Trial Title: The protective effect of melatonin in patients under carotid endarterectomy Protocol: Investigators recruited eligible Han Chinese participants (aged 40-80) who were diagnosed with carotid stenosis and had indications for carotid endarterectomy (CEA). Participants were excluded if participants had undergone surgical contraindication, or cardiovascular disease, or cerebral infarction within 3 months, or psychiatric disorders, or cancer, or pregnant, or lactating, or taking antipsychotic drugs during perioperative period. Participants were randomly divided into groups of CEA with oral melatonin, CEA with oral placebo, and CEA with blank control. Participants in groups of CEA with oral melatonin and CEA with oral placebo took melatonin orally 6mg/day or placebo from 3 days before operation to 3 days after operation. Blood samples (about 3 milliliter) were taken at baseline, 6h after the operation and 24h after the operation. Through a series hospital clinical laboratory and related ELISA kits to detect endothelial cell injury and ischemia reperfusion in serum markers. Superoxide dismutase (SOD), malonaldehyde (MDA), S100B protein, endothelial nitric oxide synthase (eNOS), nuclear erythroid 2-related factor 2 (Nrf - 2), Interleukin 6 (IL-6), nuclear transcription factor κB p65 (NF-κB p65) were included for analysis to verify whether melatonin have protective effect in patients under carotid endarterectomy. This trial is approved by the Ethical Committee of Peking Union Medical College Hospital (No ZS 1057). All participants completed a questionnaire and signed an informed consent document. Otherwise, participants 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.