Background Sleep apnea syndrome(SAS),including obstructive sleep apnea (OSA) and central sleep apnea (CSA), is highly prevalent in heart failure (HF), and it is associated with a poor prognosis. However, it remains underdiagnosed and maybe a missed therapeutic target.The aim of the study was to study the prevalence and the characteristics of SAS in HF patients and its association with left ventricular (LV) systolic function.Method It was a cross-sectional study done over a period of six months, including 80 patients with stable HF. Polygraphy and echocardiography were performed for all patients. The diagnostic threshold of SAS was set at an AHI?5/h. The patients were stratified into two groups based on LV ejection fraction LVEF: HF with reduced LVEF (HFrEF,LVEF < 50%,n=55) and HF with preserved LVEF (HFpEF,LVEF ?50%,n=25). Results Patients with HFpEF were older (66.7±9.9 versus 60.4±8.6 years,p=0.004) with a higher male sex ratio (6.8 versus 1.3, p=0.002). The anthropometric data were similar in both groups. The overall prevalence of SDB in our study was high at 83% with a predominant type of SAS being OSA (59%).There was no difference between the two groups for SAS symptoms,except for restless sleep, which was more frequent in HFpEF (64% versus 37%, p=0.025).No significant difference in SAS prevalence and severity was found between the two groups.CSA was more frequent in HFrEF (31% versus 8%, p=0,008) and OSA was more frequent in HFpEF (68% versus 54%, p=0.036).Minimal SpO2 was  lower in HFpEF (82±8.7 versus 85±7.5%, p=0.046) and the snoring index was higher. Conclusion Our study showed a high prevalence of occult SAS in both types of HF mainly OSA in HFpEF and CSA in HFrEF reflecting its severity.