No significant effect modification by commuting frequency was observed for commuting time and psychological distress: a cross-sectional study in Japan

Background Long commuting times have been associated with poor mental health, but whether commuting frequency modifies this association remains unclear. Japan, where round-trip commuting times are relatively long compared with other countries, provides a suitable setting to examine this question. Given the post-COVID-19 diversification of commuting patterns, we hypothesized that the association between commuting time and psychological distress is stronger at higher commuting frequencies. Methods We conducted a quota-sampled web-based cross-sectional study of 3922 Japanese workers in November 2025. Psychological distress was assessed as a binary outcome. The exposure variables included round-trip commuting time, commuting frequency, and the interaction between commuting time and commuting frequency. Prevalence ratios were estimated using Poisson regression with robust standard errors, with missing data handled by multiple imputation (30 datasets). Results Workers with round-trip commuting times of 60–99 min (adjusted prevalence ratio (aPR) 1.11, 95% confidence interval (CI): 1.01, 1.23) and ≥100 min (aPR 1.23, 95% CI: 1.10, 1.38) had a higher prevalence of psychological distress than those commuting <30 min. Commuting frequency showed no significant association with psychological distress (aPR 1.04, 95% CI: 0.97, 1.12). The interaction between commuting time and frequency was not statistically significant, with the ≥100 min category yielding an aPR of 1.00 (95% CI: 0.91, 1.09). Conclusions The association of commuting time with psychological distress was significant, whereas that of commuting frequency was not consistently significant. Contrary to the hypothesis, no significant effect modification by commuting frequency was observed. Future within-person studies across diverse working settings are needed to confirm these initial cross-sectional findings.