Association Between Hour-by-Hour ICU Admission Timing and 24-h Mortality: A Single-Center Retrospective Observational Study.

Aim: Prior studies of ICU admission timing often use broad categories (day vs. night), potentially masking hour-level effects. We examined hour-by-hour ICU admission timing and mortality, with attention to nighttime periods and routine handovers. Methods: We conducted a single-center retrospective observational study in a closed ICU with 24/7 on-site intensivist coverage, using prospectively registered data from the Japanese Intensive Care Patient Database. Adults (≥ 18 years) admitted to the E-ICU between April 2017 and July 2025 were included. Admission hour (0-23) was modeled as a continuous exposure using restricted cubic splines (reference 14:00). The primary outcome was 24-h mortality and the secondary outcome was ICU mortality. Multivariable logistic regression was adjusted for demographics, APACHE II, invasive mechanical ventilation at admission, comorbidities, diagnostic categories, admission source, and post-cardiac arrest status. Bootstrap resampling was used to estimate 95% confidence intervals. Additional analyses examined handover, nighttime, and 24-h periodic patterns. Results: Among 5724 patients, 24-h and ICU mortality were 3.4% and 10.6%, respectively. Compared with 14:00, adjusted odds of 24-h mortality were higher in the early morning. The early-evening estimate was imprecise, with the 95% CI including unity. In contrast, ICU mortality showed no clear temporal pattern. Handover comparisons showed no clear mortality association; nighttime admission was not associated with higher mortality, and cosinor analysis was nonsignificant. Conclusion: Hour-by-hour analysis showed higher adjusted odds of 24-h mortality in the early morning, whereas evidence of an increase in the early evening was inconclusive. Direct comparisons of handover periods showed no clear association with mortality.