Long commuting time and mental health inequalities: Middle-income vulnerability in a 14-wave panel study

Commuting time is recognized as a social determinant of mental health; however, longitudinal studies examining vulnerability patterns across different income groups remain limited. This study investigated whether specific income groups show stronger associations between commuting time and depressive symptoms in Japan, with implications for targeted policy interventions. We conducted fixed-effects models using multiply imputed data from the Japanese Life Course Panel Survey (2007–2020) on 4568 workers (39,846 observations). Depressive symptoms were measured using the Mental Health Inventory-5. Weekly round-trip commuting time (<160, 160–299, 300–499, ≥500 min) served as the primary explanatory variable. Analyses were stratified by three income groups (low, middle, high). In the overall sample, commuting ≥500 min weekly was associated with significantly lower Mental Health Inventory-5 scores compared with commuting <160 min weekly (−0.78; 95% confidence interval [CI]: −1.50, −0.07). In stratified analyses, a significant association was observed only in the middle-income group (−1.64; 95% CI: −2.80, −0.48), with no significant associations in the low-income (−0.69; 95% CI: −2.18, 0.80) or high-income groups (−0.25; 95% CI: −1.76, 1.27). Middle-income workers were the only income group that exhibited a significant association between longer commuting times and a higher level of depressive symptoms, highlighting a distinct socioeconomic vulnerability pattern. This suggests that limited flexibility in areas such as worktime control and residential choices may create health disadvantages that are specific to this group. Policy initiatives that empower workers with flexible commuting options could help to reduce these inequalities and promote equitable health outcomes. • Fixed-effects analysis found income-specific depressive symptoms pattern. • Middle-income workers uniquely vulnerable to long commuting (≥500 min weekly). • Worktime flexibility could address commuting-related health inequalities.