Objective: The goal of the study was to investigate the role time of day plays in perioperative outcomes. The authors examined intraoperative transfusion rates throughout the day in adult cardiac surgery patients. They hypothesized that the rate of transfusion changes with later case start times in scheduled cardiac surgery. Design: Retrospective observational study. Setting: Single academic medical center. Participants: Adults undergoing cardiac surgery involving cardiopulmonary bypass. Interventions: None. Measurements and Main Results: The primary outcome was a composite variable of transfusion. The association between the time of day and the rate of transfusion was explored with a multivariate logistic regression to fit the effect of starting time as a cubic spline. There were 1,421 cases that met inclusion criteria. There were 1,220 cases that were matched for modeling. The estimated probability of a patient receiving a transfusion changed significantly with later case start times in the multivariable model after adjusting for initial hemoglobin, age, sex, height, ideal body weight, diabetes, peripheral vascular disease, stroke, chronic kidney disease, chronic obstructive pulmonary disease, duration of cardiopulmonary bypass, aortic cross clamp time, attending surgeon, and attending anesthesiologist (p = 0.032, C-statistic = 0.807, n = 1220). The estimated probability of receiving an intraoperative red blood cell transfusion increased with later case start times in the multivariable model (p = 0.027, C-statistic = 0.902, n = 1220). There was no difference in the probability of transfusion for plasma, cryoprecipitate, or platelets. Conclusions: The observed rate of intraoperative blood product transfusion changed with later case start times in a multivariable model of scheduled cardiac surgery.