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Introduction Perioperative cognitive disorders include three essential elements: the progression of a pre-existing neurocognitive lesion, the occurrence of postoperative delirium (POD) and, finally, a mild or severe postoperative neurocognitive disorder. These perioperative disorders are associated with increased morbidity and mortality. Numerous risk factors are associated with perioperative cognitive disorders, the most salient being advanced age and the type of surgery. Specifically, major surgeries and cardiac surgery with associated tissue injuries and inflammation are significant causes of these cognitive impairments. The depth of anaesthesia has recently emerged as an important risk factor for perioperative cognitive disorders, particularly in older patients. The aim of our study is to evaluate whether a reduction in anaesthesia depth is associated with perioperative cognitive disorders in patients aged over 75 years who undergo planned cardiac surgery.Methods and analysis This is a multicentric randomised controlled trial with the aim to investigate whether a lower level of anaesthesia evaluated with a Bispectral Index (35 vs 55) reduces the prevalence of POD in patients aged 75 years and over who undergo planned cardiac surgery (heart valve surgery, coronary artery bypass graft surgery, aortic surgery or combined surgery). 200 patients are planned for enrolment. The analysis evaluates the prevalence of delirium during the first 3 days after surgery with the Confusion Assessment Method for the Intensive Care Unit Scale.Ethics and dissemination The study protocol has been approved by the relevant French medical review board: the French Committee for Protection of Persons (ID: 2022-A02200-43, Comité de protection des personnes Est III). We plan to present the results at (inter)national conferences after the completion of the study and to publish them in peer-reviewed journals.Trial registration number French Ethic Committee (ID: 2022-A02200-43) and ClinicalTrials.gov Registry ( NCT05877326).