Background. We aimed to identify any association between day and time of admission to critical care and acute hospital outcome.

Methods. We conducted a cohort study using prospectively collected data from the national clinical audit of adult critical care. We included 195 428 unplanned admissions from 212 adult general critical care units in England, Wales and Northern Ireland, between April 1, 2013 and March 31, 2015 in the analysis.

Results. Hourly admission rates for unplanned admissions varied more than three-fold during the 24 h cycle. Overall acute hospital mortality was 26.8%. Before adjustment, acute hospital mortality was similar between weekends and weekdays but was significantly lower for admissions at night compared with the daytime (−3.4%, −3.8 to −3.0%; P<0.001). After adjustment for casemix, there remained no difference between weekends and weekdays (−0.0%, −0.4 to +0.3%; P=0.87) or between nighttime and daytime (−0.2%, −0.5 to +0.1%; P=0.21). Delays in admission were reported for 4.3% of admissions and were slightly more common during weekdays than weekends and in the daytime than at night. Delayed admission was associated with a small increase in acute hospital mortality, but adjusting for this did not affect the estimates of the effect of day and time of admission.

Conclusions. The day of week and time of admission have no influence on patient mortality for unplanned admissions to adult general critical care units within the UK. Ways to improve critical care and hospital systems to minimize delays in admission and potentially improve outcomes need to be ascertained in future research.