Sleep disturbances are prevalent in older patients with osteoarthrosis or fracture who are scheduled for knee or hip replacement surgery. The occurrence of sleep disturbances is associated with worse outcomes including increased risk of delirium and cardiac events, and worsened functional recovery. Dexmedetomidine is a highly selective α2-adrenergic agonist with sedative, anxiolytic, and analgesic properties. It exerts sedative effects via activating the endogenous sleep pathways and produces a state like non-rapid eye movement sleep, which is different from opioid- and benzodiazepine-induced sedation. Esketamine is a N-methyl-D-aspartic acid receptor antagonist and has been used as an anesthetic and analgesic. Recent studies showed that low-dose esketamine has anti-depressive and sleep-promoting effects. The investigators suppose that mini-dose dexmedetomidine-esketamine combined infusion at night can improve perioperative sleep quality in patients scheduled for knee or hip replacement surgery.