Heart rate variability (HRV) is commonly used within sleep and cardiovascular research, yet HRV reliability across various sleep stages remains equivocal. The present study examined the reliability of frequency- and time-domain HRV within stage II (N2), slow wave (SWS), and rapid eye movement (REM) sleep during both stable and disrupted sleep. We hypothesized that high-frequency (HF) HRV would be reliable in all three sleep stages, low-frequency (LF) HRV would be reliable during N2 and SWS, and that disrupted sleep via spontaneous cortical arousals would decrease HRV reliability. Twenty-seven participants (11 male, 16 female, 26±1 years) were equipped with laboratory polysomnography for one night. Both frequency- and time-domain HRV were analyzed in two 5-10 minute blocks during multiple stable and disrupted sleep cycles across N2, SWS and REM sleep. HF HRV was highly correlated across stable N2 (r=0.839, p<0.001), SWS (r=0.765, p<0.001) and REM (r=0.881, p<0.001). LF HRV was moderate-to-highly correlated during stable cycles of N2 sleep (r=0.694, p < 0.001), SWS, (r=0.765, p < 0.001), and REM (r=0.699, p<0.001) sleep. When stable sleep was compared with disrupted sleep, both time- and frequency-domain HRV were reliable (α>0.90, p<0.05) in N2, SWS, and REM, with the exception of LF HRV during SWS (α=0.62, p=0.089). In conclusion, time- and frequency-domain HRV demonstrated reliability across stable N2, SWS and REM sleep, and remained reliable during disrupted sleep. These findings support the use of HRV during sleep as a tool for assessing cardiovascular health and risk stratification.

Introduction Proper overnight sleep is important for autonomic nervous system function. However, less is known about the effects of daytime napping on wake autonomic regulation. In the present study, we assessed autonomic function following a daytime nap. We hypothesized that a 90-minute afternoon nap would significantly improve wake heart rate variability (HRV) and blood pressure (BP). Methods Fourteen participants (7 female, 24±1 years, 24 ±1 kg/m2) took part in the study. Subjects completed an autonomic function test after no nap (control condition) or a 90-minute nap opportunity (nap condition) on separate days using a randomized, crossover design. During the autonomic test, participants were fitted with three-lead electrocardiography (ECG), continuous beat-to-beat blood pressure (Finapres NOVA, Netherlands), and respiratory monitoring (pneumobelt). The autonomic function test consisted of 5-minutes of spontaneous breathing, 5-minutes of controlled breathing (15 breaths/min), and a 2-minute cold pressor test (CPT). Frequency-domain HRV in the low (LF, 0.04-0.15 Hz) and high frequency (HF, 0.15-0.4 Hz) components were determined via Fast Fourier transformation. Time-domain HRV was quantified using RMSSD and pNN50. Paired sample t-tests were completed between the control and nap sessions. Results Mean total sleep time (TST) for the nap session was 74±5 minutes. Contrary to our hypothesis, an afternoon nap did not change wake heart rate (HR, Control: 70±3 vs. Nap: 68±3 bpm, p = .31) or mean arterial pressure (MAP, Control: 77±3 vs. Nap: 83±3 mmHg, p = 0.70). Similarly, no differences were observed in HF (Control: 2632±628 vs. Nap: 2150±494 ms2, p = .33), LF (Control: 1702±373 vs. Nap: 1345±257 ms2, p = .20), or LF/HF (Control: 92±16 vs. Nap: 92±17%, p = .97) between conditions. RMSSD (Control: 82±12 vs. Nap: 79±11 ms, p = .723) and pNN50 (Control: 43±6 vs. Nap: 47±6%, p = .30) were not impacted by a daytime nap. Lastly, changes in HR (Control: ∆14±3 vs Nap: ∆18±3 bpm, p = .114) and MAP (Control: ∆23±4 vs. ∆Nap: 27±4 mmHg, p = .28) during CPT were not different between conditions. Conclusion An afternoon nap does not appear to significantly influence autonomic function at rest or during CPT in young healthy adults. Support (If Any)

Spectral analysis of heart rate variability (HRV) is often used as a noninvasive index of cardiac sympathetic and/or parasympathetic activity during sleep. During controlled wake recordings, the low frequency (LF) component of HRV can be unreliable within experimental sessions and against other direct sympathetic measurements. In contrast, the high frequency (HF) component of HRV has higher reported reproducibility within controlled wake sessions. It remains unknown if the frequency component of HRV is reliable across sleep cycles within individuals during stable sleep. The purpose of the present study was to determine HRV reliability during stable stage II (N2) sleep, slow wave sleep (SWS) and rapid eye movement (REM) sleep. We hypothesized that both LF and HF components of HRV would be less reliable during REM sleep than during SWS and N2 sleep. Twenty‐seven participants (11 male, 16 female, 26±1 years, 27±1 kg/m2) were enrolled in the present study. Overnight polysomnography (PSG; NATUS, Middleton, WI) and continuous two‐lead electrocardiography (ECG) were recorded during an 8‐hour sleep opportunity. The nocturnal ECG recordings were imported into a custom software (WinCPRS, Absolute Aliens, Finland) for analysis of HRV via fast‐Fourier transformation function. Autonomic cardiac activity was quantified as LF (0.04‐0.15 Hz) and HF (0.15‐0.4 Hz) HRV. Two separate stable periods of sleep (range, 5‐10 min) absent of cortical arousal were selected from each sleep stage for comparison. Raw LF and HF HRV underwent logarithmic transformation due to non‐normal distribution. Statistical analysis included bivariate correlation of HRV across differing sleep stage cycles (α = 0.05). LF HRV was significantly correlated across two independent sleep cycles in N2 (r=0.694, p<0.001, n=26), SWS (r=0.765, p<0.001, n=25), and REM sleep (r=0.699, p<0.001, n=20). Similarly, HF HRV was reliable across two independent sleep cycles in N2 sleep (r=0.839, p<0.001, n=26), SWS (r=0.765, p<0.001, n=25), and REM sleep (r=0.881 p<0.001, n=21). Taken together, both LF and HF HRV measures were reliable across stable N2, SWS, and REM sleep. Our results support the reliability of frequency domain analysis of HRV during key PSG sleep stages within a single night, but further research across nights is warranted.

Introduction Heart rate variability (HRV) is a common metric to estimate autonomic activity during sleep. Frequency-domain HRV is quantified as low (LF) and high (HF) frequency, whereas HRV time-domain indices include root mean square of successive R-R interval differences (RMSSD), and percentage of successive R-R intervals differing by more than 50ms (pNN50). Despite high HRV use during sleep, it is unknown whether sleep disturbance changes overall reliability of frequency- and time-domain HRV. The purpose of this present study was to determine whether HRV was reliable across arousal-free and arousal-containing periods of sleep. Methods Twenty-seven participants (11 male, 16 female, 26±1 years, 27±1 kg/m2) were given an 8-hour sleep opportunity, equipped with continuous two-lead electrocardiography (ECG) and overnight polysomnography (PSG). The ECG recordings were analyzed via fast-Fourier transformation for frequency-domain HRV in a custom software as LF (0.04-0.15 Hz) and HF (0.15-0.4 Hz) HRV. Time-domain HRV was quantified as RMSSD and pNN50. Two separate stable sleep periods (range, 5-10min) absent of arousals were recorded, along with two separate disrupted periods of sleep with at least one arousal were selected in stage II sleep (N2), slow wave sleep (SWS), and rapid eye movement (REM) sleep. LF and HF HRV was log10 transformed due to non-normal distribution. Statistical analysis included intraclass correlations (ICC) of HRV across the four stable and disrupted periods of sleep, with separate ICC analyses across sleep stages (α = 0.05). Results Time-domain measures (RRI, RMSSD, pNN50) were reliable across arousal-free and arousal-containing sleep cycles, for all three stages (ICC&gt;0.9, p&lt;0.05). HF HRV exhibited similar reliability patterns across N2 sleep (ICC=0.960, p&lt;0.001), SWS (ICC=0.955, p&lt;0.001), and REM sleep (ICC=0.924, p&lt;0.001). LF HRV was reliable in two stages of stable and disrupted sleep in N2 (ICC=0.903, p&lt;0.001), REM (ICC=0.907, p&lt;0.001) sleep, and trending in SWS (ICC=0.616, p=0.089) sleep. Conclusion Time- and frequency-domain HRV were reliable between stable sleep with and without cortical arousals, with the exception of LF HRV during SWS. Taken together, HRV may provide a reliable, indirect index of autonomic activity across stable and disrupted sleep. Support (If Any) Support: Support is provided by the National Institutes of Health (AA-024892; U54GM115371; P20GM103474).