Dexmedetomidine is a high selective alpha-2 adrenoceptor agonist with anxiolytic, sedative and analgesic effects. When used for sedation in mechanically ventilated patients (dose range 0.2-1.4 ug/kg/h), it reduces the consumption of other sedatives and analgesics, ameliorates the rhythm and structure of sleep, and decreases the occurrence of delirium. In recent studies of the investigators, low-dose dexmedetomidine infusion (0.1 ug/kg/h) ameliorated sleep quality; for elderly patients admitted to the ICU after surgery, prophylactic low-dose dexmedetomidine infusion reduced the incidence of delirium (for patients with mechanical ventilation: 28.8% with placebo vs. 11.5% with dexmedetomidine, P<0.0001; for patients without mechanical ventilation: 15.1% with placebo vs. 6.3% with dexmedetomidine, P=0.014).