Abstract: Objective To investigate the impact of different severities of obstructive sleep apnea-hypopnea syndrome (OSAHS) on heart rate variability (HRV) in patients with ischemia and non-obstructive coronary artery disease (INOCA). Methods A total of 161 INOCA patients who were hospitalized in Department of Cardiology at Xuzhou Central Hospital from February 2024 to October 2024 were enrolled. All patients underwent portable sleep monitoring during hospitalization. Based on their apnea-hypopnea index (AHI), the patients were divided into four groups: a simple INOCA group (n=43), an INOCA with mild OSAHS group (n=63), an INOCA with moderate OSAHS group (n=32), and an INOCA with severe OSAHS group (n=23). Their general data, sleep monitoring parameters, and HRV indicators, such as standard deviation of all normal-to-normal RR intervals (SDNN), root mean square of successive differences (rMSSD), the percentage of differences between adjacent RR intervals greater than 50 ms (pNN50), and the low frequency to high frequency power ratio (LF/HF), were collected. Differences in sleep monitoring parameters and HRV indicators among the groups were compared. The correlation between HRV indicators and sleep monitoring parameters was analyzed, and multivariate logistic regression was used to identify factors influencing HRV decline in INOCA patients. Results Significant differences were observed in SDNN and pNN50 levels among the four groups (P<0.05). However, rMSSD and LF/HF levels did not show statistical differences (P>0.05). The INOCA with moderate and severe OSAHS groups showed significantly lower SDNN levels than the simple INOCA and INOCA with mild OSAHS groups (P<0.05), with significantly lower pNN50 levels than the simple INOCA group (P<0.05). Spearman correlation analysis showed that SDNN, rMSSD, and pNN50 were negatively correlated with AHI (r=-0.32,-0.23, and -0.26; P<0.01). Multivariate logistic regression analysis indicated that OSAHS is an independent risk factor for reduced SDNN in INOCA patients (P<0.05). Conclusions Moderate to severe OSAHS in INOCA patients may lead to severe sleep-related respiratory disturbances and hypoxia, resulting in decreased heart rate variability.