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 is a common breathing disorder that can lead to organic brain injury, prevent memory consolidation, and cause other adverse mentalrelated complications. Brain activity while sleeping during respiratory events is related to these dysfunctions. In this study, we analyzed variations in electroencephalography (EEG) signals before, during, and after such events. Absolute and relative powers, as well as symbolic transfer entropy (STE) of scalp EEG signals, were calculated to unveil the activity of brain regions and information interactions between them, respectively. During the respiratory events, only low-frequency power increased during rapid eye movement (REM) stage (δ-band absolute and relative power) and N1 (δ-and θ-band absolute power, δ-band relative power) sleep. But absolute power increased in lowand medium-frequency bands (δ, θ, α, and σ bands), and relative power increased mainly in the medium-frequency band (α and σ bands) during stage N2 sleep. After the respiratory events, absolute power increased in all frequency bands and sleep stages, but relative power increased in medium and high frequencies. Regarding information interactions, the β-band STE decreased during and after events. In the γ band, the intrahemispheric STE increased during events and decreased afterward. Moreover, the interhemisphere STE increased after events during REM and stage N1 sleep. The EEG changes throughout respiratory events are supporting evidence for previous EEG knowledge of the impact of sleep apnea on the brain. These findings may provide insights into the influence of the sleep apnea-hypopnea syndrome on cognitive function and neuropsychiatric defects.

Sleep apnea hypopnea syndrome (SAHS) is an independent risk factor for various cardiovascular diseases. Electrocardiogram (ECG) features such as the RR, PR, QT, QTc, Tpe intervals and the Tpe/QT, Tpe/QTc ratios are used to predict and study cardiovascular diseases. It is not clear whether regular patterns of PR and Tperelated features across sleep stages exist in SAHSs or healthy controls nor whether sleep stages affect the shortand long-range influences of respiratory events on ECG indices. We enrolled 36 healthy controls and 35 patients with SAHS in our study and analyzed the abovementioned ECG features. In the healthy controls, a significant regularity existed in these indices across sleep stages, which were weakened or disturbed in the patient group, especially the Tpe-related features. The differences between the patients and healthy controls were generally consistent across all sleep stages: patients had smaller RR, PR, QT and Tpe/QTc values, but larger QTc, Tpe and Tpe/QT values. After filtering the short-range influence of respiratory events, the differences in most features remained highly significant, except the QT interval. In the patient group, respiratory events decreased RR and PR intervals in most sleep stages and increased the Tpe-related features' values in deep sleep stages. These results may aid in the study of the relationships among SAHS, sleep disorders and cardiovascular diseases.

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Introduction Sleep apnea/hypopnea syndrome (SAHS) can change brain structure and function. These alterations are related to respiratory event-induced abnormal sleep, however, how brain activity changes during these events is less well understood. Methods To study information content and interaction among various cortical regions, we analyzed the variations of permutation entropy (PeEn) and symbolic transfer entropy (STE) of electroencephalography (EEG) activity during respiratory events. In this study, 57 patients with moderate SAHS were enrolled, including 2804 respiratory events. The events terminated with cortical arousal were independently researched. Results PeEn and STE were lower during apnea/hypopnea, and most of the brain interaction was higher after apnea/hypopnea termination than that before apnea in N2 stage. As indicated by STE, the respiratory events also affected the stability of information transmission mode. In N1, N2, and rapid eye movement (REM) stages, the information flow direction was posterior-to-anterior, but the anterior-to-posterior increased relatively during apnea/hypopnea. The above EEG activity trends maintained in events with cortical arousal. Conclusions These results may be related to the intermittent hypoxia during apnea and the cortical response. Furthermore, increased frontal information outflow, which was related to the compensatory activation of frontal neurons, may associate with cognitive function.