The investigators designed a prospective observational study to investigate the association of perioperative plasma melatonin levels with postoperative delirium and general anesthesia. The investigators included 33 patients required intensive care more than 48 hours after sevoflurane anesthesia in an academic tertiary care hospital. The investigators measured plasma melatonin levels and assessed postoperative delirium using Confusion Assessment Method for the intensive care unit (CAM-ICU). The investigators compared perioperative plasma melatonin levels between patients with and without delirium, and assessed associations of patients' demographics and anesthesia with plasma melatonin levels.