## 4 Discussion  
The study findings reveal that heart rate variability (HRV) is a crucial indicator of vital physiological processes related to athletes' recovery and fitness. In this study, HRV was measured using HRV4 Training, a validated mobile application utilizing photoplethysmography (PPG) technology. This method has been shown to be highly accurate and comparable to traditional electrocardiography (ECG)-based measurements, making it a reliable tool for long-term physiological monitoring in elite athletes [32]. Sleep quality is the most significant factor, underscoring the need for regular monitoring and improvements of sleep patterns. In this study, sleep quality was evaluated using a standardized subjective questionnaire, which captured the participant's perceptions of restfulness, disturbances, and overall recovery. While this method offers valuable insights into perceived sleep experiences, it does not account for objective physiological parameters, such as sleep duration or architecture. Nonetheless, previous research has demonstrated that both subjective and objective indicators of poor sleep are linked to parasympathetic inhibition and sympathetic activation, reducing HRV values [34; 35; 36; 37]. Our findings align with previously published studies. [PERSON] et al. [26] have examined how sleep quality impacts HRV in elite swimmers, confirming that lower sleep quality adversely affects HRV. [PERSON] et al. [38] compared both subjective and objective assessments of sleep quality with morning HRV values in healthy adults. Their findings indicate that higher-quality sleep, measured by both methods, was associated with increased morning HRV values. Furthermore, they observed that nocturnal HRV values were less reliable sleep-quality indicators than morning HRV measurements. However, none of the studies measured HRV for as many days as our study. Existing research on the relationship between sleep duration and HRV has produced mixed results. Some studies indicate that sleep deprivation causes parasympathetic inhibition and sympathetic activation, which appear as reduced HRV [39; 40]. These findings highlight the complex interaction between sleep parameters and the regulation of the autonomic nervous system and emphasize the need for further investigation of the mechanisms underlying these relationships [41]. However, a study by [PERSON] et al. [31] compared HRV in volunteers who slept for 5 consecutive nights for more than 7 h with those who slept for 5 consecutive nights for less than 5 h, which really happens to professional athletes. Our findings support previous studies showing that sleep duration has little effect on HRV [35; 38]. In a study by [PERSON] et al. [27], participants maintained their usual sleeping patterns, making this finding more comparable to our research than a study conducted by [PERSON] et al. [31], mentioned in the previous paragraph, which involved sleep restriction over five consecutive nights. It is important to note that the population measured in the study by [PERSON] et al. [27] consisted of medical staff, which cannot be directly compared to elite athletes. Previous research has shown similar average sleep durations, which, along with low variability, generally suggests that adults are getting adequate sleep based on current guidelines [42]. We found that the menstrual cycle significantly influences HRV, reflecting the dynamic interplay between hormonal fluctuations and the regulation of the autonomic nervous system. Several research studies have consistently [12; 43; 44; 45] shown that HRV changes throughout the menstrual cycle, with higher values noted during the follicular phase than in the luteal phase. This pattern is primarily attributed to fluctuations in estrogen levels, which modulate parasympathetic activity. Elevated estrogen concentrations during the follicular phase enhance vagal tone, leading to increased HRV, whereas the decline in estrogen during the luteal phase is associated with reduced parasympathetic influence and lower HRV.