biostudies-literatureUnknown9(2)Morioka N<h4>Importance</h4>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.<h4>Objective</h4>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).<h4>Design, setting, and participants</h4>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 ApJAMA network opene2558235https://www.ebi.ac.uk/biostudies/studies/S-EPMC12936879biostudies-literatureHospital Nurse Understaffing and Patient Mortality, Readmission, and Length of Stay.PMC12936879Miyawaki AMorioka NFushimi KGriffiths PSaville CMoriwaki MfalseHospital Nurse Understaffing and Patient Mortality, Readmission, and Length of Stay.<h4>Importance</h4>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.<h4>Objective</h4>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).<h4>Design, setting, and participants</h4>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 Ap2026-01-01T00:00:00Z2026 Feb2026-07-16T22:03:36.154Z2026-07-11T03:09:28.544ZS-EPMC129368794173947510.1001/jamanetworkopen.2025.58235