Improvement in patient safety is a high-priority issue of great social import. Several studies have reported that most adverse events are due to errors of hospital staff, and emphasized the necessity of promoting countermeasures against medical errors. Root cause analysis（RCA）has been implemented to prevent such errors. However, the retrospective and qualitative nature of RCA is a limitation in scientific analyses examining causal relationships. We showed that prospective design path analysis using structural equation modeling（SEM）model for both direct and indirect effects enabled statistical exploration of root causes and estimation of their impact on the outcome. Our findings suggested such analysis to be useful in devising countermeasures against medical errors. The SEM model constructed in this study suggested that one of the potential root causes was sleep disturbance. We conducted a randomized crossover study whether or not brief bright light（BL）exposure on workday mornings can improve health, performance and safety in nurses with rapidly rotating shifts. Significant improvements were noted in the BL periods compared with the non-BL periods for self-assessed sleepiness at 10:00 on day-shift days, self-assessment of night sleep for day-shift days and for fatigue. Mean response time evaluated using the psychomotor vigilance task test（PVT）showed significant improvement in the BL periods compared with the non-BL periods. The frequency of perceived adverse events and near misses was also lower in the BL than in the non-BL periods,but not significantly so,possibly indicating that the one-month observation period was too short to achieve any significant success. Despite our study’s limitations,we have effectively demonstrated the potential for preventing medical error risk among night-shift workers.