Background: Existing studies have yielded conflicting results regarding the relationship between the time of occurrence of out-of-hospital cardiac arrests and the associated outcomes. We examined whether the one-month survival rate for out-of-hospital cardiac arrests differed depending on whether the cardiac arrest occurred during the day or night. Further, we examined whether this rate differed when comparing the period succeeding the 2005 International Resuscitation Guidelines (2006–2010) with that following the 2010 guidelines (2011–2015). Method: Using data from the All-Japan Utstein Registry for 2006–2015, adult out-of-hospital cardiac arrest patients whose collapse was witnessed and for whom the collapse-to-hospital-arrival interval was shorter than 120 min were included in this study. Patients were categorized in terms of whether their arrest occurred during the post-2005- or post-2010-guideline period. The primary measure was the one-month survival with a favorable neurological outcome. Results: Of 481,624 cases analyzed, 20% occurred at night. For both guideline periods, nighttime out-of-hospital cardiac arrests were associated with significantly lower one-month survival rates than daytime incidents (used as a reference; adjusted odds ratio: 0.69 and 0.63, 95% confidence interval: 0.65–0.73 and 0.60–0.65, and P < 0.001 and <0.001 for the 2005 and 2010 guideline periods, respectively). Conclusions: One-month survival with a favorable neurological outcome was significantly lower for patients who experienced nighttime out-of-hospital cardiac arrests, compared to daytime out-of-hospital cardiac arrests. This could be addressed by improving cardiopulmonary resuscitation training for bystanders and expanding and improving nighttime emergency medical services.
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