 Methods This is a prospective observational study. Patients are recruited from the Department of NeurorehabilitationTBI Unit, Rigshospitalet, Denmark. This study will include patients above 18 years of age with severe acquired brain injury admitted for highly specialized neurorehabilitation. Patients must have a low level of consciousness defined as either vegetative or minimally conscious state. Five days of continuous heart rate monitorization at a sample rate of 256 Hz will start 40 days after the initial injury using the ePatch system. This study has been approved by the local ethics committee in the capital region of Denmark and oblige to the principles of the Helsinki Declaration. All data will be depersonalized and the Danish data protection agency has approved this study. 