Analysis of Sleep Stability and Cardiorespiratory Coupling Using PPG Signal

The main purpose of this study is to use an easy method to analyze users sleep stability. Nowadays, lots of wearable devices have already used photo-plethysmography signal (PPG) to measure physiological characteristics of sleep. So we choose PPG signal to evaluate sleep stability. And we compare the Cardiorespiratory Coupling (CRC) result with cyclic alternating pattern (CAP) in order to find out the correlation of sleep stability. In our study, the PPG data recorded from Polysomnography (PSG). And our study only uses PPG signal to detect the heartbeat interval (NNI) and PPG derived respiratory signal (PDR). So we do not need to wear airflow sensor to measure the characteristics of respiration. The CAP stages in our study are divided into 3 types (NCAP, CAP, and WREM) according to the level of sleep stability. Then we use Welch’s method to calculate the power spectral density (PSD) of NNI and PDR. When we coupling them together, we’ll get high frequency band power energy (EHF, f =0.1~0.4 Hz), Low frequency band power energy (ELF, f =0.01~0.1 Hz), and ELF/ EHF (Lo/Hi ratio). The foregoing are the characteristic we compare with CAP stages. We have 21 subjects which were divided into 3 groups: 5 normal subjects, 6 mild and moderate subjects, and 10 severe subjects. And the results show that the normal subjects do not have an obvious characteristic to compare with CAP (ACC_(PPG,NOSA)=38.37 %). Next, we analyze the sleep apnea subjects. The total accuracy of sleep apnea subjects is not a good result (ACC_(PPG,OSA)=45.31 %). But we find the detection of stage non-cyclic alternating pattern (NCAP) have a nice performance (ACC_(PPG,NCAP,OSA)=75.15 %). In order to use the detection of NCAP to determine sleep stability. We use NCAP time divided by total sleep time (NCAP Ratio). Finally, we find the result of NCAP Ratio shows a negative correlation with CAP Rate. So we think this method might have the potential to evaluate sleep stability. Compared to the previous research, the method of PPG’s measurement is easier than ECG signal. So we think this PPG method might be a better choice for Home Healthcare in the future.