LAUSR

STUDY OBJECTIVES The ability to generate slow waves (SW) during non-rapid eye movement (NREM) sleep decreases as early as the 5 th decade of life, predominantly over frontal regions. This decrease may concern prominently SW characterised by a fast switch from hyperpolarised to depolarised, or down-to-up, state. Yet, the relationship between these fast and slow switcher SW and cerebral microstructure in ageing is not established. METHODS We recorded habitual sleep under EEG in 99 healthy late-midlife individuals (mean age =59.3±5.3y; 68 women) and extracted SW parameters (density, amplitude, frequency) for all SW as well as according to their switcher type (slow vs. fast). We further used neurite orientation dispersion and density imaging (NODDI) to assess microstructural integrity over a frontal grey matter region of interest (ROI). RESULTS In statistical models adjusted for age, sex, and sleep duration, we found that a lower SW density, particularly for fast switcher SW, was associated with a reduced orientation dispersion of neurites in the frontal ROI (p = 0.018, R²β* =0.06). In addition, overall SW frequency was positively associated with neurite density (p = 0.03, R²β* =0.05). By contrast, we found no significant relationships between SW amplitude and NODDI metrics. CONCLUSIONS Our findings suggest that the complexity of neurite organisation contributes specifically to the rate of fast switcher SW occurrence in healthy middle-aged individuals, corroborating slow and fast switcher SW as distinct types of SW. They further suggest that the density of frontal neurites plays a key role for neural synchronisation during sleep.