## 4 Discussion  
In this study, we found that sleep stage, sleep position, AHI, and oxygen saturation might be more strongly associated with SB episodes than the patients' characteristics, and that longer sleep time in the supine position might be more significantly associated with SB episodes in patients with OSAS. The AHI in patients without SB episodes was higher than in such patients without SB episodes in both the supine and nonsupine positions during sleep in this study. We found that low AHI was more likely to be linked to SB episodes in OSAS, similar to previous reports [11,14]. One of the hypotheses relating to SB episodes is that they might protect against OSAS by protruding the mandible and restoring airway patency [14,22]. However, this mechanism may not be adequate to prevent the airway from collapsing in cases of severe OSAS [14]. Higher oxygen saturation in the study patients was more likely to cause SB episodes in this study. We postulate that when the AHI is high or when the oxygen saturation level is low in patients with OSAS, the mechanisms involved in opening the mouth to breathe are facilitated preferentially over those used to close the mouth for SB episodes, to resolve or compensate for apnea or hypopnea, and to improve oxygen supply. Other mechanisms might be involved in a high AHI, such as excessive respiratory efforts and increased respiratory rate [14]. Therefore, we suggest that a low AHI might induce SB episodes to restore the airway and conversely that a high AHI might reduce or prevent them. However, further studies are needed to verify our hypothesis and determine how high an AHI is associated with reducing or preventing SB episodes. The total sleep time and sleep time in the supine position were longer in these patients with SB episodes than in those without them. The longer the total sleep time, the more opportunities there are for SB episodes to occur. The association of sleep time in the supine position with SB episodes is consistent with a previous report [23]. Here, we found that a sleep time in the supine position of \(>\)280 min might be associated with SB episodes in these patients. Differences in occlusal contact patterns of teeth according to sleeping position might also be associated with SB episodes [24,25]. Our study did not include the subjects' dental conditions; therefore, it was impossible to investigate the association between SB episodes and the occlusal contact pattern of teeth according to sleeping positions. Well-designed and prospective studies examining tooth occlusal contact patterns will be needed to explore such possible associations. SB episodes can occur during any sleep stage, such as in NREM-like stages N1 and N2 and during REM sleep, or during arousal [26,27]. In this study, a more prolonged N2 sleep phase was linked to more SB episodes. Some general population studies have reported that SB decreases with age [28,29,30]. There are also reports that SB is not associated with gender in adults [26,31], although SB seems to be more prevalent in men [32]. [PERSON] et al. suggested that diabetes mellitus and male gender might be associated with SB in cases of OSAS [14]. In our study, however, SB episodes in patients with OSAS were not associated with the patients' characteristics, including age, gender, or the presence of hypertension or diabetes mellitus. However, genders were not equally represented in this study. Previous clinic-based and community-based studies have consistently reported a higher prevalence of OSAS in men than in women [33,34]. Because we compared cases with and without SB episodes among patients with OSAS rather than in healthy controls, this explains why there were more men than women in the composition of patients. However, we found that SB episodes were not associated with gender. Therefore, we postulate that the small number of female subjects in this study was associated with the effect of OSAS rather than with SB episodes.