Patients were randomized to receive sufentanil 0.05ug/kh/h + dexmedetomidine 0.1ug/kg/h (DEX group) or sufentanil 0.05ug/kg/h + saline (control group) for 48h. The primary outcome was the total sleep time during the first three days after surgery. The secondary outcomes included pain score, daily sufentanil consumption, time to functional recovery, length of hospital stay and side effects.