Delirium is common complications in postoperative critically ill patients. Although the mechanism of postoperative delirium is not well understood, there are several studies to report alternation of plasma melatonin levels after general anesthesia and suggest the link between delirium and abnormal tryptophan-serotonin-melatonin metabolism. However, there are few studies to assess the association of perioperative plasma melatonin levels with postoperative delirium and/or general anesthetics.