Insomnia partially mediates the association between ADHD traits and depressive symptoms in Japanese nursing students

Nursing students with attention-deficit/hyperactivity disorder (ADHD) traits may struggle with demanding academic and clinical training, potentially increasing the risk of insomnia and depressive symptoms. This study examined whether insomnia statistically mediates the association between ADHD traits and depressive symptoms in this population. A cross-sectional survey was conducted with 131 nursing students (96.9% female; mean age = 20.4 ± 2.3 years) at two Japanese universities in late October–November 2018, prior to the COVID-19 pandemic. Participants completed measures of ADHD traits (ASRS v1.1 Part A), depressive symptoms (PHQ-9), and insomnia (Athens Insomnia Scale). A bootstrapped mediation analysis (5,000 resamples), adjusted for age, sex, and sleep/circadian factors (sleep duration, chronotype, social jetlag), was performed. ADHD traits showed a weak positive correlation with insomnia (r = 0.29, p < 0.001) and a moderate positive correlation with depressive symptoms (r = 0.44, p < 0.001); insomnia and depressive symptoms were moderately correlated (r = 0.54, p < 0.001). Insomnia partially mediated the association between ADHD traits and depressive symptoms: the indirect effect was significant (β = 0.105, 95% bootstrap CI [0.024, 0.194]; Sobel z = 2.49, p = 0.013), accounting for approximately 26% of the total effect, while the direct effect remained significant (β = 0.293, p < 0.001). A Monte Carlo post-hoc power analysis indicated that statistical power for the indirect effect was 0.82. These findings, interpreted within the constraints of a cross-sectional design, identify insomnia as a candidate mechanism linking ADHD traits to depressive symptoms and suggest that integrating sleep health screening and targeted sleep-focused interventions into nursing education may offer a feasible and scalable preventive approach to supporting student mental health within higher-education support systems.