Sleep disturbances frequently occur in elderly patients after major surgery; and their occurrence are associated with worse outcomes including increased incidence of delirium. Previous studies showed that, for elderly patients admitted to the ICU after non-cardiac surgery, low-dose dexmedetomidine infusion improved to some degree the quality of sleep and reduced the incidence of delirium. The investigators hypothesize that, for elderly patients after major non-cardiac surgery, dexmedetomidine supplemented analgesia can also improve the sleep quality. The purpose of this randomized controlled pilot study is to investigate the impact of dexmedetomidine supplemented analgesia on the sleep quality in elderly patients after major non-cardiac surgery.