The incidence of postoperative atrial fibrillation (POAF) after on-pump cardiac surgery remains high, at around 30%. POAF increases the risk of cardiac decompensation, stroke, acute myocardial infarction, and death, resulting in increased morbidity and mortality, hospital length of stay, and cost of patient management. Episodes of POAF are usually paroxysmal and asymptomatic, increasing the risk of developing permanent AF at five years by 4 to 5 times. POAF occurs between 3 and 4 days after cardiac surgery, mainly when the patient is hospitalized in a surgical ward without heart rate monitoring as opposed to critical care, where the patient benefits from continuous rhythmic monitoring. The diagnosis of POAF is therefore made with the help of a 12-lead electrocardiogram (ECG) when the patient presents clinical symptoms and when the medical staff notes a significant variation in heart rate. However, many patients with episodes of asymptomatic POAF have a higher risk of stroke and mortality than those with symptomatic POAF. Faced with this public health problem, the development of tools for diagnosing AF is in full swing, mainly the marketing of smartwatches (SWs) that allow for the performance of 1-lead ECG. SW is also equipped with algorithms to analyze heart rate variability and diagnose asymptomatic atrial fibrillation (AF) episodes. The European Society of Cardiology (ESC) and the European Association for Cardio-Thoracic Surgery (EACTS) recommend the use of smartwatches to detect AF, in particular, to reduce the economic impact of AF. The aim of the study is to diagnose POAF within the first five days after patient discharge from the critical care unit for the cardiac surgery department.