Terminate the study because it is difficult to recruit the subject.

Delirium in older patients is associated with increased mortality and health care costs. And delirium is frequent in intensive care unit (ICU). Circadian rhythm alteration is one of the most important risk factor for developing delirium. In this study, investigators will investigate the effect of administration of melatonin on postoperative delirium in elderly ICU patients.

Randomly selected patients of the melatonin group are given 5mg of melatonin in the evening from the day before surgery to the the 5th postoperation day. In contrast, patients in the control group are given placebo.

Primary outcome of this study is the incidence of postoperative delirium measured by ICD-SC (Intensive Care delirium Screening Checklist). And secondary outcome includes intra-procedural hemodynamics measured by amount of used vasopressors, heart rate and blood pressure, total amount of sedatives, total amount of analgesics, duration of mechanical ventilation, duration of ICU stay.

Thus, investigators perform this study to investigate the relationship between delirium and circadian rhythm.

Inclusion Criteria:

• older than 50 years old
• patients who admitted to ICU after surgery

Exclusion Criteria:

• Subjects are ineligible if they have cognitive dysfunction,
• disabling mental change disorder,
• alcohol addiction,
• dementia,
• cerebrovascular accident,
• transient ischemic attack,
• carotid artery stenosis,
• autoimmune disease,
• patients who are unable to communicate or speak Korean,
• emergent operation