This is a randomized controlled trial to determine the safety and efficacy of Sitagliptin in patients with stress hyperglycemia or mild diabetes following cardiac surgery. Patients will receive Sitagliptin or placebo. Sitagliptin may be of particular use in this patient population due to the effects on hepatic glucose production (a major feature of SH), safety (lack of contra-indications for heart failure or renal failure and no hypoglycemia), and tolerability. The secondary objective of this study is to determine whether the management of patients with persistent insulin requirements following cardiac surgery differs among patients with stress hyperglycemia or mild diabetes.