Sleep is indispensable for humans to maintain normal life activities. Sex and individual differences in sleep patterns and quality of sleep cannot be ignored. Nevertheless, the overall population generalities and sex-or individual-differences in cerebral cortical functional connectivity (FC) during sleep have not been well described. Here, we evaluated the characteristic patterns of FC based on whole-night sleep electroencephalography (EEG) recordings. An improved weighted phase lag index (WPLI) algorithm was applied to obtain the FC in delta (0.5-4Hz), theta (4-8Hz), alpha (8-12Hz) and beta band (12-32Hz). FC strength, short-term stability and inter-regional imbalance of FC were studied. We found that the variations in FC-related parameters among sleep stages had overall population commonalities, and these parameters also showed stage-and frequency band-dependent sex differences. With the deepening of norapid eye movement (NREM), increased delta and beta FC strength were observed. Rapid eye movement (REM) showed weaker FC strength, higher FC stability, and higher anterior-posterior FC anisotropy than NREM in beta band. Meanwhile, females exhibited higher sleep EEG synchronization and higher delta FC stability in deep NREM sleep than males. Moreover, the dominant hemisphere in terms of FC did not show group generality or stage-and frequency-dependence. Our results add to the understanding of sleep staging function and may provide clues to sex differences in sleep patterns and quality as well as the prevalence and clinical manifestations of sleep-related illness. Short-term stability offers a new perspective in analyzing FC, which cannot be ignored.INDEX TERMS Overall population generalities, sex differences, sleep stages, sleep EEG, functional connectivity.
Sleep apnea hypopnea syndrome (SAHS) is an independent risk factor for cardiovascular diseases. However, the pathophysiology between them is not yet clear. This paper seeks to understand how respiratory events impact the cardiovascular system by heart rate variability. We compared the differences between successional pathological respiratory events (PR) and pure normal respiration (NR) during sleep. The transitions between normal and pathological respiration (TR) were also analyzed. Thirteen patients who suffered moderate or severe SAHS were enrolled in this study. The results demonstrate that the beat-to-beat interval (RR interval) mean value and sample entropy are significantly lower during PR than during NR. RR interval standard deviation, the power of very low frequency (VLF) and low frequency (LF), total power, and the low frequency/high frequency (LF/HF) ratio were significantly larger during PR than during NR. However, the high frequency (HF) power was not significantly different between normal and pathological respiration. Additionally, the trends during TR also supported these significant differences. The results indicate that during pathological respiration, as the heart rate and its volatility increase, the complexity of its rhythm decreases. We conclude that the energy of the autonomic nervous system rapidly increases during pathological respiration, especially at the beginning. The HF power does not significantly change to modulate the heart rhythm, but the activity of the sympathetic nervous system will significantly increase, resulting in the imbalance of the LF/HF ratio. In addition to these findings, this paper discusses the influence of arousal on these indices during TR.
Sleep stage also has an effect on this neuromodulatory mechanism. These findings may help clarify the relationship between SAHS and cardiovascular disease.
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