Flatter diurnal cortisol slopes amplify the association between daily stress and somatic symptoms.

BACKGROUND: Daily somatic symptoms, such as fatigue and headaches, are common among adults and can forecast health decline. Flatter diurnal cortisol slopes (DCS), indicating a dysregulated hypothalamic-pituitary-adrenal (HPA) axis, are linked to physical health impairments. Whether DCS moderate the within-person relationship between daily stressors and somatic symptoms, and whether stress buildup further influences this association, remain unclear. METHOD: Middle-aged community adults (N = 1621) completed daily diary interviews and offered salivary cortisol samples over four days, producing 6484 person-day observations. Three-level generalized linear mixed-effects models using a Tweedie distribution examined main effects, two-way and three-way interactions among DCS, within-person daily stressor exposure, and stress-buildup burden on daily somatic symptom counts. Daily stress was assessed for both stressor frequency and severity; the average stressor severity was assigned a priori as the key definition. RESULTS: Within-person stressor exposure was linked to higher same-day somatic symptoms across all definitions. Flatter DCS were associated with greater somatic symptom burden when stress was defined as the daily stressor count (β = 0.217). DCS significantly moderated the count-derived stress-symptom relationship (β = 0.019) but did not moderate the severity-average relationship (β = -0.017). Moreover, stress buildup moderated this cortisol-related pattern (three-way interaction: β = -0.043), but this outcome was sensitive to the imputation approach and should be interpreted with caution. CONCLUSION: DCS was related to differential somatic reactivity to everyday stress, such that flatter DCS was connected with stress-somatic symptom coupling. Contextual stressors and physiological regulation likely jointly affect daily somatic health among adults.