Background: Sleep apnea (SA), a modifiable risk factor in - atrial fibrillation (AF), is associated with worse outcomes in AF. We aimed to assess the prevalence and severity of SA in patients with AF, and, subsequently, to assess the positive predictive value (PPV) of moderate to severe SA by a home-monitoring device in comparison to cardio-respiratory monitoring (CRM) in consecutive patients with AF. Methods: This cross-sectional study recruited unselected patients with AF without known SA from an out-patient clinic at Department of Cardiology, Herlev-Gentofte University Hospital. Participants underwent four consecutive nights of sleep-recording with the home-monitoring device NightOwl™ (NO). Moderate SA was defined as an Apnea-Hypopnea Index (AHI) of 15–29 and severe SA as ≥ 30 AHI. Participants with moderate to severe SA was offered CRM for validation of the diagnosis. Results: We included 126 patients with AF with a median age of 68 (interquartile range: 60–75) years, 42 (33 %) women, 70 (56 %) hypertension, 61 (48 %) hyperlipidemia and 49 (39 %) heart failure. NO detected severe SA in 36 (29 %) of patients with AF, moderate SA in 35 (28 %), mild SA in 45 (36 %) and no SA in 10 (8 %). Of 71 patients with moderate to severe SA by NO, 38 patients underwent CRM and the PPV of NO was 0.82 (31/38) to diagnose moderate SA and 0.92 (22/24) to diagnose severe SA by CRM. Conclusion: Moderate to severe SA by NO was highly prevalent in patients with AF without known SA. A home-monitoring device such as NO could be an easy and feasible SA screening tool in patients with AF.