How were sleep spindle patterns linked to memory and sleep apnea in older adults?

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Sleep spindles are a hallmark of non-REM sleep and play a fundamental role in memory consolidation. Changes in these distinct electroencephalographic oscillations are emerging as sensitive biomarkers for neurodegenerative diseases of ageing. Understanding the clinical presentations associated with spindle changes may help clarify their functional role and the pathophysiology of sleep and neurodegenerative disorders.

Researchers used a data-driven approach to examine sleep, memory and default mode network connectivity phenotypes associated with sleep spindle architecture in older adults, whose mean age was 66 years. Participants were recruited from a specialist clinic for early diagnosis and intervention for cognitive decline; some showed mild cognitive deficits on neuropsychological testing.

The sample included 88 people who underwent memory assessment, overnight polysomnography and resting-state functional MRI (fMRI). Researchers applied k-means cluster analysis to fast spindle density, spindle duration and spindle amplitude.

The analysis identified three clusters. Cluster 1 had preserved spindle architecture, with higher fast spindle density and longer spindle duration; Clusters 2 and 3 had altered spindle architecture. Both Clusters 2 and 3 were characterized by reduced memory, while Cluster 3 was also characterized by nocturnal hypoxemia associated with sleep apnea.

Resting-state fMRI analysis confirmed a relationship between default mode connectivity and spindle architecture, although the direction of the relationship differed across clusters. Overall, the findings show diversity in spindle architecture among older adults, associated with clinically meaningful phenotypes including memory function and sleep apnea. They suggest that default mode connectivity at rest while awake can be associated with sleep spindle architecture, but that this association is highly dependent on clinical phenotype. Establishing relationships between clinical and neuroimaging features and spindle changes may advance understanding of the bidirectional relationships between sleep changes and neurodegenerative diseases of ageing.