## 3 Results  
Figure 2 shows a hypnogram in a representative subject at the onset of sleep. As the sleep stage changes from wake to NREM sleep, large ABM disappears, a sharp HF peak appears in R-R  
interval power spectrum, and Hsi increases above 70%. Figure 3 is an all-night hypnogram in another representative subject. While Hsi is below 70% during wake and REM sleep periods, it exceeds 70% in synchrony with NREM sleep periods. Figure 2: Hsi of a healthy male subject at the onset of sleep. From the top, hypnogram, actigraphic body movement (BM), R-R interval (RRI) power spectrum, and Hsi. As the sleep stage changes from waking (W) to NREM (N1, N2, and N3), large BM disappears, a sharp peak appears in high frequency band (0.15-0.4 Hz) in RRI power spectrum, and Hsi (area under the curve) increases above 70%. In the bottom panels, \(\omega\) is the frequency race to integrate the spectral power around the high frequency peak. PSD = power spectral density. From 141 subjects, a total of 11,636 segments of data were obtained, including 2261 (17.1%) awake, 6891 (52.2%) NREM, 2484 (18.8%) REM, and 1578 (11.9%) transitional segments. The average values of the HRV indices, scaling exponents, and ABM for each stage are shown in Figure 4 and Table 2. As shown in Figure 4, Hsi was greater during NREM stage than during wake, REM, and transitional stages (\(p<0.0001\)) and ABM was greater during wake stage than NREM and REM stages (\(p<0.0001\)). As shown in Table 2, VLF power, LF/HF, \(\alpha_{30-60}\), and \(\alpha_{60-300}\) were lower and HF and HF peak power was greater during NREM stage than during wake and REM stage, heart rate, (\(p<0.0001\) for all). significant effect on Hsi, although Hsi was greater in females (least-square mean [SE], 65.9 [0.7]%) than males (64.1 [0.4]%, \(p=0.038\)). ROC curve analysis for the dominant stage per 5-min segment revealed that the performance of Hsi for discriminating NREM from wake and REM stages was significantly greater than those of other HRV indices, scaling exponents, and ABM (Table 3 and Figure 5). Particularly, the performance of Hsi was greater than that of HF peak power, indicating the importance of the sharpness rather than the height of HF peak. When Hsi \(>70\%\) was used as a cutoff, Hsi detected NREM sleep with 77% sensitivity, 80% specificity, 79% accuracy, and a Cohen's kappa coefficient of 0.57 (95%CI, 0.55-0.58). The discriminant performance of Hsi was preserved (AUC = 0.87) even in the separated analysis of 19 subjects with moderate-to-severe SDB (AHI, 26.0 [21.7-44.5/h]). Since Hsi was considered to reflect the regularity of respiration frequency, we compared the discriminant performance of Hsi calculated from the R-R interval and respiration itself. Like the Hsi of R-R interval, the Hsi of respiration was greater during NREM stage than during wake, REM, and transitional stages (\(p<0.0001\), Table 2). The discrimination performance of Hsi of respiration (AUC = 0.85), however, was comparable to that of Hsi of R-R interval (Table 3 and Figure 5). ROC curve analysis for the sleep stage per 30-s epoch revealed similar results (Table 4). Although the AUC of Hsi was lower than that for 5-min segment analysis, it was still significantly greater than those of other HRV indices, scaling exponents, and ABM. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Variable** & **AUC (SE)** & **95\%CI** & **P vs. Hsi** & **Cut off** & \ \begin{tabular}{c} **Sens** \\ **\%** \\ \end{tabular}  & \begin{tabular}{c} **Spec** \\ **\%** \\ \end{tabular}