BACKGROUND Sleep loss in attending physicians has an unclear effect on patient outcomes. In this study, we examined the effect of medical care provided by physicians after midnight on the outcomes of their scheduled elective procedures performed during the day. METHODS We conducted a population-based, retrospective, matched-cohort study in Ontario, Canada. Patients undergoing 1 of 12 elective daytime procedures performed by a physician who had treated patients from midnight to 7 a.m. were matched in a 1:1 ratio to patients undergoing the same procedure by the same physician on a day when the physician had not treated patients after midnight. Outcomes included death, readmission, complications, length of stay, and procedure duration. We used generalized estimating equations to compare outcomes between patient groups. RESULTS We included 38,978 patients, treated by 1448 physicians, in the study, of whom 40.6% were treated at an academic center. We found no significant difference in the primary outcome (death, readmission, or complication) between patients who underwent a daytime procedure performed by a physician who had provided patient care after midnight and those who underwent a procedure performed by a physician who had not treated patients after midnight (22.2% and 22.4%, respectively; P=0.66; adjusted odds ratio, 0.99; 95% confidence interval, 0.95 to 1.03). We also found no significant difference in outcomes after stratification for academic versus nonacademic center, physician's age, or type of procedure. Secondary analyses revealed no significant difference between patient groups in length of stay or procedure duration. CONCLUSIONS Overall, the risks of adverse outcomes of elective daytime procedures were similar whether or not the physician had provided medical services the previous night. (Funded by the University of Toronto Dean's Fund and others.).