Background: Despite clinical optimization, many chronic heart failure (CHF) patients remain symptomatic with dyspnea and poor quality of life. Study Objective: While oxygen therapy is prescribed in severe cases, the actual prevalence of different patterns of hypoxemia is unknown. Methods: We analyzed 183 stable CHF patients with optimized medical treatment in the “MARS” database. The patients underwent cardiorespiratory sleep recording and complete daytime pulmonary function tests including arterial blood gases. Results: This prospective cohort was predominately male (86.3%) with a mean age of 67.3 years (59.3; 75.7) and a mean BMI of 26.7 kg/m² (23.7; 31.1). The patients were mainly in NYHA classes II and III with a mean left ventricular ejection fraction of 38%. 102 (55.61%) patients had ischemic cardiomyopathy with multiple comorbidities, and 64 (35.06%) had airflow obstruction. 8 (4.37%) patients had hypoxemia both day and night, and 151 (82.5%) had nocturnal hypoxemia only. All but 3 patients had sleep-disordered breathing (SDB), and either obstructive (59%) or central sleep apnea (39%) with a mean apnea-hypopnea index of 29.59/h (16.48; 48.27), an oxygen desaturation index of 27.09/h (14.09; 45.25), time below 90% saturation of 18 min (2; 64), and a mean nocturnal saturation of 93% (92; 94). Univariate analysis found nocturnal hypoxemia was associated with higher BMI and NT-proBNP levels. In multivariate analysis, only sleep apnea severity (p < 0.0001) and diurnal PaO₂ remained significant. Conclusion: Most stable CHF patients suffer from nocturnal hypoxemia, while daytime hypoxemia is relatively rare. The degree of nocturnal hypoxemia depends on the severity of SDB. Hypoxemia phenotyping and severity could help better evaluate the need for appropriate therapy in CHF patients.