The goal of this observational study is to learn how exercise-induced irisin protects the brain. It also explores irisin's effect on postoperative delirium (POD) in older adults. The main questions it aims to answer are:

Does daily activity link to POD? Does irisin play a mediating role in this process? Researchers will enroll active and inactive adults aged 65 and older. All will have surgery under spinal anesthesia. The researchers will measure irisin levels in spinal fluid. They will track POD and other complications for five days after surgery.

Inclusion Criteria:

• Aged ≥ 65 years;
• Classified as ASA physical status I to III;
• Scheduled for non-cardiovascular surgery under spinal anesthesia or combined spinal-epidural anesthesia;
• Expected surgical duration of at least 2 hours;
• Voluntarily provide written informed consent and agree to participate in this study.

Exclusion Criteria:

• Significant change in exercise status within the past 7 days due to injury or other reasons;
• Contraindications to neuraxial anesthesia;
• Unable to assess cognitive function or Montreal Cognitive Assessment (MoCA) score < 23;
• Preoperative diagnosis of delirium;
• History of traumatic brain injury or neurosurgery;
• Long-term opioid use or history of alcohol abuse;
• Planned surgery cancelled or changed (including intraoperative events that significantly alter the procedure or duration);
• Change in anesthesia method.

Patients scheduled for elective surgery at Peking University People's Hospital.