We present an instrumented bed for unobtrusive, non-contact monitoring of cardiac and respiratory activity. The system presented here is based on the principle of ballistocardiography (BCG), and measures cardiopulmonary vibrations of the body by means of an electromechanical foil (EMFi) attached to the mattress. Using our system, a clinical study with 13 participants was conducted to assess the BCG’s ability to distinguish atrial fibrillations from normal sinus rhythms. By computing a time-frequency representation of the recorded signals based on parametric autoregressive estimators, we can show clear qualitative differences between normal and arrhythmic BCG episodes. The same distinctive features could also be observed when applying our method to a simultaneously recorded reference ECG. Our results suggest that ECG and BCG both contain the same basic information with respect to the presence of atrial fibrillations, and that a bed-mounted BCG sensor can indeed be used to detect atrial fibrillations.