Hospital Nurse Understaffing and Patient Mortality, Readmission, and Length of Stay.

Importance: Although higher nurse staffing has been linked to better patient outcomes, most studies rely on aggregate measures, leaving the effects of deviations from ward- and shift-level norms (daily and per shift) unclear. Clarifying these effects can inform ward-level staffing decisions. Objective: To investigate the association between nurse understaffing, relative to typical ward staffing levels, during the 24-hour period, day shift, and evening and night shift and patient risk for in-hospital mortality, readmission, and longer length of stay (LOS). Design, Setting, and Participants: This retrospective cohort study used claims data and shift-ward rosters from 82 wards in 9 hospitals in Japan. Included patients were aged 20 years or older and hospitalized between April 1, 2019, and March 31, 2020. Analyses were performed from March 20 to November 12, 2025. Exposure: Nurse understaffing was defined as average nurse hours per patient-day below each ward's annual median, assessed for the 24-hour period, day shift, and evening and night shift. Main Outcomes and Measures: The primary outcomes assessed were in-hospital mortality, 7-day and 30-day readmissions, and LOS. Propensity score matching (PSM) and multilevel models adjusted for confounding. Results: This study included 77 289 hospital admissions. Patients had a mean (SD) age of 69.3 (15.1) years, 57.2% were male, and 53.2% were admitted for surgery. Of these admissions, 28 846 matched pairs were included in the in-hospital mortality analysis and 27 907 matched pairs were included in the readmission analysis. After PSM, patients exposed to nurse understaffing during the 24-hour period and day shift had higher in-hospital mortality rates (3.1% vs 2.8% and 3.2% vs 2.8%; both P = .02); those exposed to understaffing during the 24-hour period also had higher 30-day readmission rates (11.2% vs 10.5%; P = .01), whereas those exposed to understaffing during the day shift had higher 7-day readmission rates (2.3% vs 2.1%; P = .04). Understaffing during the evening and night shift was not associated with these outcomes. LOS after PSM was longer when understaffing occurred during the 24-hour period (mean [SD], 14.6 [16.3] vs 13.8 [16.5] days), day shift (14.7 [16.4] vs 13.7 [16.2] days), or evening and night shift (14.1 [16.2] vs 13.6 [16.4] days) (all P < .001). Conclusions and Relevance: In this cohort study, nurse understaffing during the 24-hour period (specifically during the day shift) was associated with increased risk of in-hospital mortality, hospital readmission, and longer LOS. These findings suggest that close attention to daily staffing levels and prompt action to address understaffing may help improve patient outcomes. Nurse managers should routinely monitor deviations from this benchmark to address immediate needs and enable rapid adjustments through feasible strategies.