This study aims to investigate whether heart rate variability (HRV), a marker of autonomic nervous system activity, can predict the occurrence of rebound pain after total knee arthroplasty (TKA). Rebound pain is defined as a sudden and intense pain episode (NRS ≥7) that typically arises after the resolution of peripheral nerve blocks used for postoperative analgesia. Patients undergoing elective unilateral TKA under spinal anesthesia with peripheral nerve blocks will be included. HRV will be measured both before and after surgery using a chest-worn heart rate monitor. Pain levels, analgesic consumption, sleep quality, and patient satisfaction will also be recorded. The primary goal is to determine whether perioperative HRV values can serve as a predictive biomarker for rebound pain. Secondary outcomes include the relationship between HRV and pain intensity, opioid use, sleep quality, and length of hospital stay. The findings may contribute to developing individualized pain management strategies for TKA patients.