Unconstrained Heart Rate Estimation System in Bed

It is widely known that autonomic nervous system (ANS) is one of the indices to indicate the wakefulness of person. Therefore measuring the biosignals relating with the ANS during sleep is required to evaluate sleep quality. There are many researches that heart rate derived from the electrocardiogram (ECG) is good index to show the sympathetic or parasympathetic activities. To assess the quality of sleep, polysomnagraphy is used as golden standard but subjects have to attach many of sensors to their body and it is not only complicate, but also influence their sleep. In this study, to measure the autonomic nervous activities, we introduce newly developed system using load-cell. The sensors installed bottom of the bed measure the ballistocardiogram, mechanical vibration of heart. Ballistocardiogram is one of the nonintrusive methods to measure the heart rates unconstrainedly. By comparing with reference signal, ECG using Ag-AgCl electrodes, we validated our system with the four subjects. The results show comparison of LF/HF ratio, one of the heart rate variability, measured from the load-cells and Ag-AgCl electrodes for the each sleep stage, deep sleep, light sleep and REM sleep. It shows the different pattern for the light and deep sleep but contrary to our expectation, REM sleep doesn´t indicate the highest value. LF/HF ratio measured from both methods have no significant differences.