In this study, we improved upon a high-input impedance monitoring device comprising capacitive sheet electrodes placed under a bed sheet, aiming to detect all ECG waves including T waves. To accomplish this, we widened the passbands for ECG and respiratory movement (RM) signals. However, because this widening may lower noise tolerance, we substituted conventional front-end buffers with buffers able to discharge DC potential. Further, the backside of the modified electrode was doubly shielded by using the electric potential of the buffer’s output and ground. We also introduced a virtual midpoint circuit to realize stable ECG and independent RM measurements of the chest and abdomen with the goal to detect obstructive sleep apnea in future applications. We tested our device on seven healthy adults during a six-hour sleep experiment. In all subjects, we detected T waves and observed ECG and RM signals in supine and lateral positions. In experiments in which respiratory rate was changed at regular intervals, the RM signal was synchronized with the simultaneously measured reference signal. The sensitivity and accuracy of R waves averaged over all subjects were 93.2% and 92.9%, respectively. Both averaged sensitivity and accuracy for T waves were 94.5%, while averaged accuracy of chest and abdominal RM was 70.9% and 74.0%, respectively. The results for ECG signals indicate that our improved system is applicable to unconstrained monitoring of vital signs in adults. Although there is still room for improvement in RM measurements, the results suggest that our system may also be used to detect variations in breathing.