<p>Emotional inertia refers to the stability of emotional states over time and has been linked to overall well-being and mental health. This pilot study examines overnight emotional inertia, that is the carry-over of emotions across the sleep period, from evening to morning, and particularly its relationship with evening cortisol levels. 26 female primary caregivers reported their emotional states and provided saliva samples for cortisol measurement each morning and evening over two weeks. Multi-level models were used to regress morning emotions on evening emotions and cortisol levels. Thereby, evening cortisol levels were found to moderate the overnight inertia of negative emotions, with higher cortisol levels associated with stronger persistence of negative emotions from evening to morning. These findings suggest that elevated evening cortisol levels enhance the carry-over of negative emotions into the next day, potentially linking stress-related physiological processes with overnight emotional inertia. These findings provide preliminary evidence that evening HPA-axis activity may be associated with the overnight persistence of negative affect, although replication in larger and more heterogeneous samples is needed.</p>