INTRODUCTION: Sleep deprivation, which is a common complication in the intensive care unit (ICU), is associated with delirium and increased mortality. Sedation with gamma-aminobutyric acid agonists (propofol, benzodiazepine) results in significant disturbance of the sleep architecture. Dexmedetomidine is a lipophilic imidazole with an affinity for α2-adrenoceptors and it has sedative and analgesic properties. It has been reported to enhance sleep efficiency, thus sedate while preserving sleep architecture. METHODS AND ANALYSIS: Thirty consecutive patients are planned to be included, at the Department of Anesthesia and Intensive Care at the Hospital of Southwest Jutland, Denmark. The study is a double-blinded, randomised, controlled trial with two parallel groups (2:1 allocation ratio). Screening and inclusion will be done on day 1 from 8:00 to 16:00. Two 16 hours PSG (polysomnography) recording will be done starting at 16:00 on day 1 and day 2. Randomisation is performed if the first recording is of acceptable quality, otherwise the patient is excluded before randomisation. Dexmedetomidine/placebo will be administered during the second recording from 18:00 on day 2 to 6:00 on day 3. PRIMARY ENDPOINT: Improvement of total sleep time and sleep quality of clinical significance determined by PSG. SECONDARY ENDPOINTS: Sleep phases determined by PSG. Daytime function and delirium determined by Confusion Assessment Method-ICU. Alertness and wakefulness determined by Richmonde Agitation Sedation Scale. The objective is to compare the effect of dexmedetomidine versus placebo on sleep quality in critical ill mechanically ventilated patients. ETHICS AND DISSEMINATION: The trial investigate the potential benefit of dexmedetomidine on clinically relevant endpoints. If a beneficial effect is shown, this would have a large impact on future treatment of mechanically ventilated critically ill patients. Publication in peer-reviewed journal are plannedand the study has been approved by the National Committee on Health Research Ethics (ID:S-20180214). TRIAL REGISTRATION NUMBER: EudraCT (2017-001612-11DK) and Danish National Committee on Health Research Ethics (ID:S-20180214). The study related to pre-results.