## 1 Introduction  
The obstructive sleep apnea is a disturbance characterized by a fragmented and nonrestorative sleep, which results from episodes of either complete or partial collapse of the airway, accompanied by decrease in oxygen saturation or arousal from sleep [1]. In spite of the fact that its epidemiology is monitored from 90s only, currently it is observed that the prevalence of the obstructive sleep apnea may be estimated as 3-7% of general population, with predisposing factors listed as: older age, male sex, excessive body mass, family history of obstructive sleep apnea, being after menopause (especially for women without hormone-replacement therapy), craniofacial abnormalities, tobacco smoking, and alcohol consumption [2]. If untreated, the obstructive sleep apnea may lead to both short-term and long-term health consequences; short-term consequences include excessive sleepiness during day, fatigue, noturia, morning headache, irritability, and memory impairment, while long-term consequences include increased risk of cardiovascular diseases, metabolic disorders, cognitive impairment, and depression [3]. As a result, it also influences the quality of life, reducing both physical and mental functions [4]. In the treatment of obstructive sleep apnea, the positive airway pressure (PAP) is the gold standard; it is commonly applied as the continuous PAP (CPAP), which stabilizes the upper airway with constant positive pressure while using a mask interface, but also as the bi-level PAP, which uses various pressures for the inspiratory and expiratory cycle (for inhalation and exhalation) [5]. The systematic review and meta-analysis by [PERSON] et al. [6] revealed that in patients with obstructive sleep apnea and hypertension, the CPAP therapy reduces blood pressure, especially nighttime blood pressure. Similarly, the meta-analysis by [PERSON] et al. [7] indicated that while applying CPAP for more than 4 h per night, the risk of major cardiovascular events is reduced by 57%. Nonetheless, it must be also considered that for PAP or CPAP treatment, the adherence is crucial as it is indicated that in case of low adherence, the potential benefits of PAP therapy are not obtained [8]. In a large retrospective study, it was observed that low adherence is associated with increased hospital all-cause and cardiovascular-cause readmission [9]. Within the other retrospective study, it was observed that this therapy may result in a decrease in healthcare resource utilization, though this reduction is observable in patients who adhere to therapy [10]. The PAP or CPAP treatment adherence may differ between countries [11]; at the same time, other factors influencing adherence include hypertension, sleep posture, and apnea-hypopnea index (AHI) reduction obtained during treatment [12]. Moreover, barriers to obtaining required adherence include: inability to afford a PAP or PAP device (if not covered); perception of symptom reduction, resulting in belief in no need for treatment; and dissatisfaction with treatment [13], arising from physical barriers (including mask leaks, and dry throat and nose) and psychological barriers (including anxiety, clautrophobia and, insomnia) [14]. Taking this into account, in obstructive sleep apnea the other therapeutic methods are also applied and are sometimes even more important; examples include surgical treatment [15] and other methods applied mainly as supportive therapies, such as oral appliances, weight loss, or positional therapy [5]. Among the listed methods, the weight loss is indicated to be beneficial and recommended for all overweight or obese individuals with obstructive sleep apnea due to the fact that the association between excessive body mass and obstructive sleep apnea is obvious; thus, the potential of body mass reduction is significant [16].