Non-linear association of social jetlag with depressive symptoms and suicidal ideation: mediation and age/sex heterogeneity in an exploratory cross-sectional study

Objectives This study examined the nonlinear associations between social jetlag (SJL), depressive symptoms, and suicidal ideation. It also tested whether inflammatory markers and depressive symptoms mediated these associations. Stratified analyses by age and sex were performed. Methods Data came from 5,838 adults in the 2017–2020 National Health and Nutrition Examination Survey. Restricted cubic splines were used to model nonlinear relationships. Mediation analyses assessed the roles of an inflammatory burden index (CRP and neutrophil‑to‑lymphocyte ratio) and depressive symptoms. All analyses were stratified by age group and sex. Results A near U‑shaped association emerged. Zero SJL corresponded to the highest depressive symptom scores. Mild‑to‑moderate SJL (> 0 to < 2 hours) was linked to lower scores, whereas severe SJL was linked to elevated scores. This pattern was strongest in young adults (20–39 years). Among women, moderate SJL was associated with a 65% reduction in the odds of suicidal ideation, compared with no SJL. Mediation analyses showed that depressive symptoms accounted for 25.2% of the SJL–suicidal ideation association. The inflammatory burden index did not mediate this association. Conclusions Mild‑to‑moderate SJL may have certain beneficial associations with mental health outcomes in specific subgroups. The overall SJL–mental health relationship is nonlinear. The mediating pathway operates primarily through depressive symptoms, not systemic inflammation.