## 4 Discussion  
The main goal of the present study was to investigate the stability of the severity of sleep-disordered breathing by determining the characteristic parameters and the stability of the polygraphic patterns during repetitive examinations (two nights). The importance of a reliable, widely accessible, cost-effective examination method for sleep-disordered breathing is underlined by the data, where 80-90% of patients who suffer from OSAS are undiagnosed [17]. The question arises about the effect of daily variation of sleep-disordered breathing on the severity and therapeutic approaches. Due to the daily variation of our activity, sleep amount, etc., one night of sleep examination might be insufficient to describe the exact severity. In addition to the former, the first-night effect of the sleep examination can also modify the result of the exact severity of the sleep-disordered breathing [11]. The first-night effect may be more pronounced for PSG due to the number of electrodes, non-home environment, and continuous monitoring by sleep technicians. Two nightly examinations can improve the reliability of the result. [PERSON] and colleagues [18] compared one and two nights of PSG examination for the diagnosis of sleep-disordered breathing. They found a significant night-to-night variation of the polysomnographic respiratory variables and concluded that 13% of the patients might benefit from the second examination. Similar variability was also measured in the study of [PERSON] [11]. Fifteen percent of patients showed significant variation between the two nights, causing a 6% of loss of severe patients with one recording. In contrast with the former investigations, [PERSON] and colleagues [19] concluded, that only a mild first night effect can be found, with no effect on the diagnosis of sleep-disordered breathing. The gold standard for the diagnosis of sleep-disordered breathing is the polysomnographic examination [12; 14; 20], which requires a complex technical background and polysomnographic technologist. The cost of the examination is high, and the availability is limited. Despite the standards and guidelines, the reliability of polysomnography is questionable in some circumstances [21; 22]. The investigations of [PERSON] [23] revealed a negative polysomnographic examination of a patient with clinically positive symptoms, and a home sleep test proved the presence of sleep-disordered breathing. The cost-effective form of OSAS diagnosis is the polygraph-based home sleep test. The indications and contraindications are defined by the AASM guideline [12; 14; 20]. Depending on the applied sensor set, many forms are used. The accuracy of the polygraphic examination can be improved by the use of an extended sensor set (including body position sensor and two belts for breathing effort) [12]. In this study, we examined the accuracy of the repetitive polygraphic examination. Our results revealed good correlation of the AHI, ODI and T90 between the two consecutive nights for 100 examinations. Determining the severity range-specific correlations, we found the highest correlation for AHI in the severe group. The results were confirmed by the severity rank-specific statistics (Wilcoxon and Spearman). The correlation for ODI and T90 varied from moderate to high in all severity groups. This finding proves the reliability of the PG for the diagnosis of sleep apnea syndrome, especially in the severe form. Our examination provides further evidence for the appropriate diagnostic procedure for obstructive sleep apnea syndrome in the case of the severe category. In given circumstances (positive complaints, witnessed apneas, and in the presence of comorbidities--representing the high probability of severe disease), polygraphy can be a good choice for an adequate diagnosis. Our examination showed the night-to-night variation of the severity in 25% of examinations. Both increase and decrease of the severity rank were found during the second examination.