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Abstract Purpose Postoperative delirium (POD) is a usual complication after total hip/knee replacement, which may be affected by sleep characteristics. However, up to now, preoperative sleep characteristics have not been evaluated as risk factors of POD. The relationship between self‐reported sleep characteristics and POD in patients has been investigated in this study. Patients and Methods We recruited 495 cognitively intact individuals in the final analysis from the Perioperative Neurocognitive Disorder and Biomarker Lifestyle study. Sleep characteristics were tested by the Pittsburgh Sleep Quality Index (PSQI). Mini‐mental state examination was applied to assess preoperative mental status of patients. Postoperatively, we used confusion assessment method and memorial delirium assessment scale to evaluate the incidence of POD and POD severity, respectively. The cerebrospinal fluid (CSF) levels of T‐tau, P‐tau, Aβ40, and Aβ42 were detected by enzyme‐linked immune‐sorbent assay before the operation. Logistic regression, multiple linear regression, and mediation effects were performed to analyze the relationship between self‐reported sleep characteristics and POD. Results POD was detected in 11.31% (56/495) of the patients, with logistic regression analysis showing that daytime dysfunction, P‐tau, and T‐tau were risk factors of POD, and Aβ42 was a protective factor of POD. Multiple linear regression analysis confirmed that daytime dysfunction was positively correlated with P‐tau in patients with POD. Meanwhile, compared to the patients with no postoperative delirium, the CSF levels of P‐ and T‐tau were higher in patients with POD. Furthermore, mediation analysis showed that it was probable that daytime dysfunction mediated POD through P‐tau (proportion: 12.90%) partially. Conclusion Daytime dysfunction is a risk factor of POD preoperatively. To sum up, CSF P‐tau protein might partially mediate the influence of daytime dysfunction on POD. Clinical trial registration This study was registered at Chinese Clinical Trial Registry (ChiCTR2000033439).