BACKGROUND: We aim to examine diurnal and weekday variations in citizen responder availability and intervention at out-of-hospital cardiac arrest (OHCA) resuscitation.

METHODS AND RESULTS: We included confirmed OHCAs where citizen responders were activated by a smartphone application in the Capital Region of Denmark between September 1, 2017 and August 31, 2018. OHCAs were analyzed by time of day (daytime: 07:00 AM-03:59 Pm, evening: 4:00-11:59 PM, and nighttime: 12:00-06:59 AM) and day of week (Monday-Friday or Saturday-Sunday/public holidays). We included 438 OHCAs where 6836 citizen responders were activated. More citizen responders accepted alarms in the evening (mean 4.8 [95% CI, 4.4-5.3]) compared with daytime (3.7 [95% CI, 3.4-4.4]) and nighttime (1.8 [95% CI, 1.5-2.2]) (P

CONCLUSIONS: Citizen responders were more likely to accept OHCA alarms during evening and weekends, with the highest proportion of responders arriving before Emergency Medical Services in the evening. However, there was no significant difference in delivering cardiopulmonary resuscitation or early defibrillation among cases where citizen responders arrived before Emergency Medical Services.