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Objective Out-of-hospital cardiac arrest (OHCA) has low survival rates with worse outcomes at night due to delayed emergency medical services (EMS) response, resource limitations and workforce fatigue. Since randomised trials are unfeasible, all-comers registries provide essential data to bridge evidence gaps and improve EMS protocols.Design Retrospective observational study using propensity score matching.Setting National EMS registry and death registry data from Poland, cases from September to November 2022.Participants Of 2388 eligible patients, cases were grouped by time of cardiac arrest (on-hours: 7:00–18:59; off-hours: 7:00–18:59 AM) and matched 1:1 using propensity scores, yielding 1194 pairs.Primary and secondary outcome measures Primary: return of spontaneous circulation (ROSC) and 30-day survival.Secondary: EMS response time.Results Our findings revealed significant disparities in OHCA outcomes between day and night shifts. ROSC rates were notably lower at night (20.9% vs 34.8%; p=0.01); however, no difference in 30-day survival was observed (8.3% vs 8.1%; p=0.94). Furthermore, EMS response times were significantly longer during nighttime hours (median and IQR): 12.4 (7.4–14.6) versus 11.2 (6.2–13.5) (minutes); p=0.01.Conclusions Patients with OHCA during off-hours experienced longer EMS response times and significantly lower rates of ROSC as compared with daytime hours. No difference in 30-day survival was observed between groups. Potential contributors include reduced staffing, fatigue and logistical delays. System-level changes in EMS scheduling and workforce planning might help to reduce time-of-day-related disparities in OHCA outcomes.Trial registration number Clinical Trials ID: NCT03130088; Post results