Closed-loop acoustic stimulation enhances the initiation and persistence of slow oscillation trains during sleep in elderly adults

Slow oscillations (SOs) during non-rapid eye movement (NREM) sleep are critical for multiple brain functions. Aging is associated with reduced SO amplitude and density, as well as disrupted temporal organization, characterized by more isolated events and shorter rhythmic SO trains. Here, we investigated whether closed-loop acoustic stimulation (CLAS) could restore rhythmic SO activity in elderly adults. Subjects underwent an adaptation and an experimental night and were assigned to a control or stimulation group. We analysed SO temporal organization using metrics previously introduced by our group (Capurro et al., 2026), including the proportions of isolated, consecutive, and train-start SOs and SO train length. Analyses were performed locally around stimulation periods and globally across NREM sleep. CLAS reorganized local SO dynamics by reducing isolated SOs and increasing consecutive SOs, train initiation, and train persistence. Global analyses showed weaker but consistent effects, mainly characterized by longer SO trains. Importantly, these changes occurred without increasing overall SO density, suggesting that CLAS promoted sustained rhythmic SO activity rather than simply increasing SO occurrence. These findings suggest that CLAS may mitigate age-related alterations in SO dynamics and highlight temporal organization metrics as a complementary framework for assessing stimulation efficacy, particularly in systems affected by stimulation latency and jitter.