Endogenous melatonin is produced by the pineal gland at night under normal conditions and regulates the sleep-wake cycle. Artificial light administered at night suppresses melatonin production and sleep disturbances are accompanied by abnormal melatonin secretion such as phase delay. Therefore, dramatic disturbances of endogenous rhythms in intensive care unit patients have remarkable effects on melatonin production. In addition to its physiological roles in regulating sleep patterns, melatonin has been demonstrated to provide antiinflammatory effects in experimental models. Although some previous studies have investigated the circadian pattern of melatonin in intensive care unit patients, the investigators think that the present study is the first one that will assess the effects of controlling noise and light on melatonin and inflammatory response after major abdominal surgery.