LAUSR

BACKGROUND Chronotype influences several physiological systems, including the immune system and the hypothalamus-pituitary-adrenal (HPA)-axis. Previous research has shown that evening chronotype is associated with adverse metabolic health outcomes and obesity. However, the exact mechanisms underlying the observed differences in metabolic function between "morning" and "evening" types remain to be explored. OBJECTIVE To investigate the relationship of chronotype with inflammatory and neuroendocrine stress markers and to explore their mediating and moderating roles in the association between chronotype and body mass index (BMI). METHODS Twenty-eight healthy young adults (50% women), mean age 23.8 ± 3.3 (SD) years, underwent a standardized laboratory stress test (Trier Social Stress Test, TSST). Concentrations of plasma C-reactive protein (CRP) at baseline and of salivary cortisol before and after the onset of the stressor were analyzed. Heart rate was measured continuously. Chronotype was assessed with the Morningness-Eveningness Questionnaire (MEQ). RESULTS Lower MEQ scores (i.e. evening tendency) were associated with higher BMI (r = -.40, p < .05), elevated CRP concentrations (r = -.42, p < .05) and higher cortisol responses to acute stress (r = -.53, p < .01). The relationship between MEQ score and BMI was mediated by CRP concentrations (β = -0.03, CI 95%: -0.08 to -0.007, p < .05). In addition, we observed a moderating effect of the cortisol stress response on this mediated relationship (β = 0.005, CI 95%: 0.0002 to 0.01, p < .05), such that the mediated relationship was stronger in individuals with a higher cortisol response. CONCLUSION Enhanced pro-inflammatory state and a higher cortisol response to stress may underlie the effect of evening chronotype on obesity risk and adverse metabolic health outcomes.