Inclusion criteria:  Aged 18 to 65 years Currently prescribed vancomycin and piperacillin/tazobactam with the presumption that therapy will be continued for at least 3 days based on a review of subject status. Because of the critical nature of starting empiric broad-spectrum antibiotics, we will allow one dose of the antibiotic combination before consent and enrollment. This strategy is necessary for the ethical conduct of the study.  Exclusion criteria:  Estimated creatinine clearance < 30 mL/min Liver impairment (liver enzymes > 3 times upper limit) Any history of allergy or contraindication to melatonin Pregnancy or breastfeeding Autoimmune disease Requiring vasopressors Requiring mechanical ventilation History of acute kidney injury in the past 30 days Inability to take oral medications Clinical evidence of significant unstable or uncontrolled illness which, in the opinion of the research team, could confound the results of the study or put the patient at undue risk.