## 4 Discussion
This nearly equal and bidirectional (improvement and worsening) change ruled out the first-night effect (false negative or milder severity) of the polygraphic examination in this study and might be based on the daily variability of the sleep instead of the first-night effect. In [PERSON]'s work [18], the difference between the two overnight tests was significant if the change in AHI between the two was greater than 5/h. This does not represent real severity changes in all cases. In contrast, in our study, we used clinically significant change (associated with a change in therapy and a change in formal severity grade). In [PERSON]'s work, a change was described in 13% of patients in the second study, whereas in our study, only 6 out of 100 cases had a change in severity grade (6%) that warranted a change in therapy. We consider that clinically significant change (requiring a change in therapy) is a more useful approach for daily practice. The majority of studies use parameters of the sleep disordered breathing (AHI, ODI, etc. ), but other variables like the TIB or pattern of examination can provide further insight into the exact pathology. In contrast with the AHI, ODI, and T90, the length of examination (TIB) revealed a moderate correlation between the two nights for all 100 pairs of examinations. The severity-specific correlation of TIB was highest in the negative group. This finding can be explained by the lack of the effect of sleep fragmentation and awakenings in the negative group. In contrast, in other severity groups, breathing pathology can modify the sleep time (TIB) day by day through sleep fragmentation and consequential awakenings. There are only limited and controversial data about the role of the nightly pattern change on the severity of sleep-disordered breathing. According to the examinations of [PERSON] [24], the time spent in the supine position was significantly higher during the first recording. In contrast, [PERSON] [25] found no differences in the supine position between the two nights. According to [PERSON]'s results [25] on gender differences in this question, women recorded by polygraph spent more time in a supine position. The examination of [PERSON] [26] can explain the higher number of supine positions during the polysomnographic examination. The patients spent more time in the supine position during the PSG examination compared to the recording without an extended electrode set. This result emphasizes that the polysomnographic recording can overestimate the severity of sleep-disordered breathing in some circumstances. These examinations are based on the change in severity due to the time spent in a given--i.e., supine--position. The consequence of the more time in the supine position can be a higher respiratory disturbance index. These data highlight the importance of night-to-night variability in the pattern of sleep-disordered severity ranking. The novelty of our study is to examine the variability of different patterns between the two recorded nights. Theoretically, the different patterns could represent different pathophysiological backgrounds. An example of this might be the pathophysiological difference between the supine-dominant (with highly anatomical origin) and REM-dependent (alteration of the regulation during REM phase) patterns. The statistical analysis of pattern changes showed good correlation between the examination nights. Pattern changes were found in 15% of 100 examinations during the second night. According to our results, the most stable patterns are the supine-dependent and continuous with phasic worsening (its polysomnographic equivalent is the continuous with REM worsening); the former is associated with high structural pathogenesis, and the latter is regulatory in origin. In 6 of the 15 cases, pattern changes influenced the severity of sleep-disordered breathing, with the consequence of potential therapeutic change.