Anterior Cruciate Ligament Reconstruction (ACLR) is a common orthopedic procedure that is accompanied by moderate to severe postoperative pain and is primarily performed on an outpatient basis.

Many different pain control treatments are used in the management of patients after ACLR, including various permutations of nerve blocks, nerve block adjuncts (NBAs), intra-articular injections, intravenous (IV), oral medications, and cryotherapy, as well as compression stockings. Despite this, no gold-standard postoperative analgesia guideline has emerged.

Melatonin (N-acetyl-5-methoxytryptamine) is a naturally-occurring hormone in the human body and offers antiemetic, analgesic, and anxiolytic effects

Inclusion Criteria:

• Patients undergoing elective unilateral arthroscopic reconstruction of the anterior cruciate ligament (ACL) under spinal anesthesia, aged from 18 to 65 years old, and who are American Society of Anesthesiologists (ASA) physical status I or II of either sex

Exclusion Criteria:

• Contraindications to neuraxial anesthesia (e.g. allergy to local anesthetics, coagulopathies, infection in the area)
• History of cardiovascular, cerebrovascular, respiratory, or psychological disorders
• Chronic pain syndrome and/ or opioid use
• Regular consumption of analgesics, corticosteroids or anticonvulsants
• A known allergy to melatonin
• Patients with diabetes mellitus, sever hypertension, liver or renal dysfunction
• Cognitive disorders (dementia, major depression)
• Circadian rhythm disorders (such as chronic fatigue syndrome and drowsiness)
• Pregnancy/ lactating
• Body mass index 35 kg/m2 or more,
• Patient refusal.