Both obstructive and central sleep apnea (OSA and CSA) may contribute to nocturnal cardiac arrhythmias (NCA). Data on the prevalence of clinically important nocturnal atrial and ventricular arrythmias in patients with heart failure with reduced ejection fraction (HFrEF) and obstructive (OSA) or central sleep apnea (CSA) are scarce. In a cohort of patients with HFrEF, how does the prevalence of NCA compare among those with OSA, CSA and those with no to mild sleep apnea? Is the severity of OSA or CSA associated with atrial and ventricular NCA? This cross-sectional analysis is an ancillary study of the ADVENT-HF trial. We compared the prevalence of NCA (excessive supraventricular ectopic activity [ESVEA], defined as premature atrial complexes [PAC] ≥30/h or supraventricular tachycardia ≥20 beats); atrial fibrillation/flutter [AF]; and >10 premature ventricular complexes [PVC/h]) on ECGs from polysomnograms of HFrEF patients between those with OSA (apnea-hypopnea index [AHI≥15]), those with CSA (AHI≥15/h) and those with no to mild sleep apnea (AHI<15, control). Compared to controls (n=76), the prevalence of ESVEA was higher in patients with OSA (n=430) and CSA (n=150), 0%, 9% and 12%, respectively. The prevalences of AF in the control, OSA and CSA groups were 9%, 17% and 27%, and of >10 PVC/h 45%, 59% and 63%, respectively. In multivariable regression analyses PAC/h was associated with OSA severity (obstructive AHI) [22.4% increase per 10 events/h (95% confidence interval: 5.2; 42.3); p=0.009], but neither obstructive nor central AHI were associated with AF or >10 PVC/h. In patients with HFrEF, the prevalences of nocturnal ESVEA, AF and PVC >10/h were higher in those with, than in those without OSA or CSA, and OSA severity was related to the burden of nocturnal atrial ectopy. Severity of OSA or CSA were not significantly related to AF or >10 PVC/h.