Rumination impedes overnight stress recovery: Findings from ecological momentary with nocturnal heart rate assessments

Speed of affective and physiological recovery from daily stress is a crucial yet understudied pathway to mental health resilience. Autonomic nervous system markers, heart rate (HR) and HR variability (HRV), are implicated in anxiety and depression disorders, though the directionality remains debated. Here, we followed N = 105 medical students (73 female, aged 23.87 ±1.81) during a year of high-stress internships, integrating questionnaires, 10,798 ecological momentary assessments, and 13,872 nights of physiological Fitbit recordings. We observed increases in nocturnal HR and decreases in HRV over time independent of microstressor exposure or concurrent mental health changes. Within-person increases in daily stress predicted higher following-morning negative affect, 28.5% of which was explained by rumination, a modifiable target for intervention. Individual differences in autonomic and affective recovery did not predict resilience. Our findings highlight the value of real-world intensive assessments to identify intervention targets to support recovery during periods of stress.