Circadian sleep regulation, cognition and brain structural-functional integrity in healthy older adults

Identifying novel factors which associate with inter-individual variability in -and can be considered protective for- cognitive decline is a promising area in aging research. Circadian rhythms -one of the most fundamental processes of living organisms- are present throughout the nervous system and act on cognition and underlying brain function and structure. Circadian rhythms shape the temporal organisation of sleep and wakefulness and evolve throughout the adult lifespan, leading to higher sleep-wake cycle fragmentation with aging. During this session, we will first revise the association between circadian sleep regulatory processes on microstructural brain integrity by focusing on rapid eye movement sleep (REMS). REMS is increasingly suggested as a discriminant sleep state for subtle signs of age-related neurodegeneration. We show that reduced circadian REMS amplitude is related to lower magnetization transfer saturation (MTsat), longitudinal relaxation rate (R1) and effective transverse relaxation rate (R2*) values in several white matter regions mostly located around the lateral ventricles, and with lower R1 values in grey matter clusters encompassing the hippocampus, parahippocampus, thalamus and hypothalamus (Deantoni et al., 2024). Our results highlight the importance of considering circadian regulation for understanding the association between sleep and brain structure in older individuals. In a next step, the impact of daytime napping, a visible manifestation of circadian sleep alteration, on cognition and brain structural-functional correlates will be highlighted. Our results suggest that increasing daytime rest observed during aging is underlined, at the neurobiological level, by an altered integrity of the wake-promoting posterior hypothalamic region (Baillet et al. 2022). Functional MRI data collected in the same population further speak in favor of reduced functional compensation, expressed by altered hemispheric asymmetry in task-related BOLD activation. This session integrates novel findings using a variety of approaches (quantitative structural MRI, functional MRI, EEG) to characterize pathways by which the sleep-wake cycle, or more specifically sleep schedules, affect cognition and underlying brain correlates during aging. The topic is of relevance, since both cognitive impairment and insufficient or mistimed sleep represent dominant health determinants across lifespan, and easy implementable interventions, potentially focused on sleep regulation, are urgently needed.