Prescription opioid misuse and its associated negative effects have become an epidemic in the United States, and post-operative opioid use contributes to this terrible problem. Alternative strategies to opioid prescribing are thus highly sought after in the post-operative setting. Importantly, sleep and pain have a bi-directional relationship, and inadequate or impaired sleep regularly occur following orthopedic operations. Melatonin is an endogenous sleep hormone that can be administered exogenously, and that has been shown to have some potential as an analgesic agent. Here, using the premise that melatonin may improve sleep and pain in the post-operative setting, the investigators propose a randomized clinical trial in 120 participants undergoing total knee arthroplasties. Patients will be randomized to receive either sublingual melatonin 5 mg or matched placebo starting on post-operative day (POD) 0 and through POD . The investigators will measure post-operative opioid usage as the primary outcome, and post-operative pain scores as a secondary outcome. The primary safety outcome will be sedation level, as measured by the Richmond Agitation Sedation Scale (RASS). Sleep will be measured objectively using wrist-worn actigraphy. Participants will be followed through POD 28, and will also have baseline data on sleep, pain, and cognition obtained prior to surgery.

Inclusion Criteria:

• Adults age 18 years and older
• Undergoing elective total knee arthroplasty (single knee)

Exclusion Criteria:

• Non-English speakers
• Individuals with dementia
• Patients with liver cirrhosis
• Patients currently taking prescription sleep aids
• Patients with long-term (greater than 3 months prior to surgery), chronic opioid use