###### Abstract  
(1) Background: Alternating interhemispheric slow-wave activity during sleep is well-established in birds and cetaceans, but its investigation in humans has been largely neglected. (2) Methods: Fuzzy entropy was used to calculate a laterality index (LI) from C3 and C4 EEG channels. The subjects were grouped according to an apnoea-hypopnoea index (AHI) for statistical analyses: Group A AHI \(<\) 15 (mild); Group B 15 \(\leq\) AHI \(<\) 30 (moderate); Group C AHI \(\geq\) 30 (severe). The LI distribution was analysed to characterise the brain activity variation in both hemispheres, and the cross-zero switching rate was given statistical tests to find the correlations with the severity of obstructive sleep apnea and sleep states, i.e., wake (W), light sleep (LS), deep sleep (DS), and REM. (3) Results: EEG brain switching activity was observed in all sleep stages, and the LI distribution shows that, for obstructive sleep apnea patients, the interhemispheric asymmetry of brain activity is more obvious than healthy people. A one-way ANOVA revealed a significant difference of switching rate among three groups (F(2,95) = 7.23, \(p\) = 0.0012), with Group C shows the least, and also a significant difference among four sleep stages (F(3,94) = 5.09, \(p\) = 0.0026), with REM the highest. (4) Conclusions: The alternating interhemispheric activity is confirmed ubiquitous for humans during sleep, and sleep-disordered breathing intends to exacerbate the interhemispheric asymmetry. EEG; brain switching; obstructive sleep apnoea; laterality index; fuzzy entropy +
Footnote †: journal:  
morning headache, observed episodes of breathing cessation during sleep and abrupt awakenings accompanied by gasping or choking [6]. The apnea-hypopnea index (AHI) is widely used to evaluate the severity of OSA. It records the average number of apnea or hypopnea events per hour. It is commonly accepted that the AHI values are categorized as: (i) normal when AHI \(<\) 5; (ii) mild sleep apnea when 5 \(\leq\) AHI \(<\) 15; (iii) moderate sleep apnea when 15 \(\leq\) AHI \(<\) 30; and (iv) severe sleep apnea when AHI \(\geq\) 30 [7]. The standard approach to diagnose OSA involves in-laboratory polysomnography (PSG), in which EEG remains the best technique for the functional imaging of the brain during sleep [8; 9]. EEG is the most common tool used in sleep research [10; 11; 12; 13; 14; 15; 16]. Recently, EEG coherence analysis has garnered considerable interests to study the functional asymmetry of the brain [17; 18; 19; 20; 21; 22]. A fundamental left-right brain switching mechanism is observed across the animal kingdom from invertebrates through to birds and mammals, including. humans. Reports show that when facing competing motion stimuli on its left and right, Drosophila would choose one stimulus at a time and then try to rotate toward its direction. This behaviour indicates that Drosophila has alternating left/right optic lobe local field potential (LFP) activities [23]. [PERSON] et al. revealed that some switching might exist in the control of birdsong timing between the left/right high vocal centre (HVC) on the timescale of song syllables or long subsyllabic elements [24; 25]. For hamsters, continuous light would separate the daily activities into two parts, with antiphase circadian oscillations in the left/right suprachiasmatic nuclei (SCNs), namely, splitting of circadian rhythms. In split hamsters, the right/left SCN oscillate 12 h out of phase with each other, and the twice-daily locomotor bouts alternately correspond to one or the other [26; 27]. Alternating left-right nostril patency in rat, rabbit, cat, dog, domestic pig, and humans [28; 29; 30; 31; 32]. Alternating unihemispheric slow-wave activity is well-established in birds [33] and aquatic mammals [34]. Existing reports show that alternating unilateral slow-wave sleep is always the most common sleep pattern for some specific species, such as marine mammals, birds, and possibly certain reptiles.