A total of 20 participants were recruited to the study. Women (n=11; mean age 34 yrs, SD 7 yrs; mean height 1.69 m, SD 0.05 m; mean weight 67 kg, SD 10 kg) and men (n=9; mean age 42 yrs, SD 8 yrs; mean height 1.80 m, SD 0.06 m; mean weight 78 kg, SD 3 kg) were healthy (eg, no disease) and nonsmokers, and did not take medication on a regular basis. The participants were fully informed about the study design and the use of measurement devices before signing an informed consent document. The study complies with the standards set by the Ethics Committee of the University of Jyväskylä, Finland.

Nocturnal recordings were taken at home during sleep. Recordings began shortly after participants went to bed and stopped once they left their bed in the morning. Before the first recording, Emfit QS’s own proprietary cellular ferroelectret sensor was placed beneath the mattress or mattress topper under chest area (Figure 1). The reference RR interval (RRI) data were recorded with Firstbeat Bodyguard 2 (BG2), an ECG-based recorder with two disposable electrodes and a sampling frequency of 1000 Hz. BG2 and its electrodes were set up on a participant’s body according to instructions in the user manual. The accuracy of BG2 was previously evaluated in laboratory protocol studies by Parak et al [13] and Bogdány et al [14], and in our unpublished study, which showed perfect agreement in the detection of RRIs (r=1.00) during a 30-minute rest period with Custo Cardio 100BT, a 12-channel ECG device (Custo med GmbH). The participants recorded the data over 3 consecutive nights, and the last recording was used in the analysis. Before measurements were taken, time synchronization was performed on the devices.

Emfit QS provides HR, the vagal-related HRV index, and the root mean square of successive differences (RMSSD) in RRIs throughout the sleep period in 3-minute epochs. If heart beat detection is disturbed due to a poor signal or artifacts any time during this period, data are not collected. The RRI data of BG2 were analyzed using the Firstbeat Sports software (Firstbeat Technologies Ltd). RRIs were checked by an artifact detection filter of the Firstbeat Sports software [15] and subsequently excluded all falsely detected, missed, and premature heart beats caused by movement artifacts or any other artifacts of unknown origin. HR and RMSSD values were calculated for each 3-minute epoch throughout the measurement. Averages of the whole night period, which were used in the analysis, were calculated from those 3-minute values. Emfit QS and BG2 data were synchronized according to the time stamp. In addition to HR and RMSSD data, the amount of missing data was calculated.

Values are expressed as mean (SD). Averages of the whole night period were calculated from the 3-minute epochs. The Gaussian distribution of the data was assessed with the Shapiro-Wilk goodness-of-fit test. Ln (natural logarithm)- transformation was applied to the RMSSD data in order to meet the assumptions of the parametric statistical analysis. The accuracy of HR and RMSSD measured by Emfit QS was evaluated by determining the amount of missing data, the mean bias (absolute and percentage), and the root mean square error (RMSE) compared with the reference (BG2). Statistical difference between the measurements of BG2 and Emfit QS was analyzed using a paired Student t test. In addition, the magnitude of the differences was expressed as effect size (ES). The difference was considered trivial when ES≤0.2, small when ES≤0.6, moderate when ES≤1.2, large when ES≤2.0, and very large when ES>2.0. A Pearson product-moment correlation and a Bland-Altman plot were used to analyze agreement between the reference and Emfit QS data. A Spearman rank correlation coefficient was calculated to investigate the correlation between the absolute differences (reference minus Emfit QS) and the average of the devices for HR and Ln RMSSD. In addition to the measures of statistical significance, the following criteria were adopted to interpret the magnitude of the correlation between measurement variables: <0.10 (trivial), 0.11-0.30 (small), 0.31-0.50 (moderate), 0.51-0.70 (large), 0.71-0.90 (very large), and 0.91-1.0 (almost perfect) [16]. Significance was accepted as P<.05. Data were analyzed using SPSS Statistics 25 software (IBM Corp).

To the best of our knowledge, this study is the first to evaluate the accuracy of BCG-based nocturnal HR and HRV measurements under real-life conditions. The main finding of this study is that BCG-based Emfit QS showed a trivial mean bias in nocturnal mean HR (1.7%, ES=0.16) and RMSSD (2.0%, ES=0.14) compared with the ECG-based reference device. In addition, the correlation coefficient showed good agreement (r=0.89-0.90) between the devices. Terbizan et al [17] suggested a minimum correlation of 0.9 for heart monitors to be clinically reliable. However, it needs to be acknowledged that a high correlation coefficient does not solely represent good agreement in all cases [18].

Choe and Cho [19] found an RMSE of 1.8 bpm (beats per minute) for HR measured over a 15-minute rest period by a piezoelectric sensor (BCG) under laboratory conditions. The error was slightly smaller compared with that in our study (2.4 bpm). In addition, Xie et al [20] reported a 0.90 bpm mean absolute error in HR detection by a BCG sensor placed under a chair during 15-minute recordings and a greater correlation (r=0.98) compared with the present study. In previous studies, the participants were instructed to avoid movement since muscular activity may cause measurement errors in BCG. In this study, the measurements were carried out at home under real-life conditions. During sleep, movement can have interfering effects on HR and HRV detection, which can explain the slightly higher error and the amount of missing data in the present study.

For valid HRV determination, consecutive heart beats must be detected with a high degree of accuracy. Shin et al [21] observed a 5% relative error and a strong correlation (r=0.97) in a time-domain analysis of HRV by BCG and ECG. Wang et al [22] observed perfect correlations (r=0.99-1.00) in HRV variables between BCG and ECG. The authors concluded that HRV can be measured reliably with BCG. In this study, the mean bias in RMSSD was smaller (2.0%) than that in the study by Shin et al [21], but the correlation was weaker (r=0.89).

The Bland-Altman plots showed that 5% (1/20) of the mean HR and Ln RMSSD values were outside the limits of agreement (LoA). Bland and Altman [23] recommended that 95% of the data points should lie within the mean difference (SD 1.96). Our results showed a proportional error in the mean Ln RMSSD determined by Emfit QS. Based on the Bland-Altman plot, a larger error can be found in smaller and larger Ln RMSSD values; it seems that Emfit QS underestimates Ln RMSSD at high HRV levels and overestimates Ln RMSSD at low HRV levels. The LoA for the mean HR is relatively narrow (SD 4.6 bpm, ~8%). It is important that LoA would be narrower than daily changes in long-term monitoring. Al Haddad et al [24] reported a ~12% day-to-day variation in resting Ln RMSSD. In the present study, the LoA for the mean Ln RMSSD was ~12%. Thus, it can be concluded that it is not greater than the day-to-day variation in Ln RMSSD; the LoA is barely acceptable. Unfortunately, this research did not answer whether the biases are stable in repetitive measurements. Reliability is crucial for long-term monitoring of stress and recovery states; if the bias varies within an individual in repetitive measurements, the true changes in daily HR and HRV cannot be detected, and thus, an accurate interpretation of cardiac autonomic regulation will be compromised. In addition, it is good to standardize sleeping and measurement conditions (eg, bed, mattress) in long-term monitoring for reducing biases.

The 3-minute averaged data showed no differences in HR at any time points during the night and few differences in Ln RMSSD (Figure 5). The differences can be explained by the greater amount of missing data of Emfit QS; it is possible that a 3-minute epoch can include only, for example, 15 seconds of Emfit QS data; yet, the system provides a value for the 3-minute epoch. Comparatively, the reference data could include the entire 3-minute data, which could explain the differences in the 3-minute values. Thus, it seems that HR and HRV values for the entire night are more adequate. The overall view of nocturnal HRV is also more important than a single 3-minute value because HRV fluctuates during sleep according to sleep phases [25,26].

Although the bias and LoA analyses appear promising, future development in the accuracy of HRV measurement is needed to decrease the amount of missing data and incorrect 3-minute values. Because of the relatively large amount of missing data in HRV by Emfit QS compared to the reference data (28% vs 1%), the HRV data do not reflect values of the whole night for every individual. Suliman et al [11] observed large differences (77%-95%) in the ability to detect BCG peaks between different algorithms during a short resting period. Thus, the development of these algorithms may decrease missing data. It could be speculated that individuals with a high amount of missing data could also have more differences between the reference and Emfit QS, but our results showed no correlation between the amount of missing data and the mean bias of the measurements. Furthermore, future studies should clarify reliability over time for measuring HR and HRV by Emfit QS.

Morning ECG measurements with HR straps are time-consuming and arduous to perform every day, and thus, compliance with regular measurements is poor [27]. Nocturnal HRV is a time-efficient method compared to morning HRV measurements taken at waking [28]. Previous studies have mainly focused on BCG measurements under laboratory conditions. This study showed the potential of Emfit QS to serve as a tool for everyday use at home to measure nocturnal HRV. Contact-free and fully automatic analysis by Emfit QS facilitates effortless daily monitoring of stress and recovery status among athletes. Furthermore, it does not require attaching electrodes on the body surface and thus does not disturb participants’ typical sleep behaviors, which is advantageous over ECG. Based on our results, Emfit QS provides HR and HRV data with an acceptable, small mean bias compared with ECG. However, due to some large errors in detecting HR and HRV in some individuals, it would be best practice to ensure accuracy by comparing initial results from Emfit QS with ECG.

This study evaluated the accuracy of BCG-based Emfit QS for measuring HR and HRV. Our results showed that Emfit QS provides HR and HRV data with an acceptable, small mean bias compared with ECG. Thus, Emfit QS can be a potential tool for the long-term monitoring of HR and HRV. However, further research is needed to evaluate the reliability of HR and HRV detected by Emfit QS.