Circadian rest/activity rhythms and change in MRI‐derived brain volumes: Differences by age in a cognitively healthy sample

INTRODUCTION: Circadian rest/activity rhythms (RARs) change across adulthood, and alterations in RARs have been associated with cognitive decline, Alzheimer's disease (AD), and dementia. Little is known, however, about associations of RARs with subsequent changes in brain structure. METHODS: We investigated cross-sectional and longitudinal associations between RARs from wrist actigraphy and AD-relevant brain volumes from magnetic resonance imaging in cognitively unimpaired adults aged ≥ 50 and whether age moderated these associations. RESULTS: In cross-sectional analyses, weaker and more fragmented RARs were associated with smaller medial temporal region volumes. Longitudinally, more regular and less fragmented RARs were associated with better maintenance of amygdala volume over time. Further, associations between RAR fragmentation and global atrophy were strongest among the oldest participants. DISCUSSION: Results link RARs with medial temporal lobe and global brain volumes and indicate that these associations may differ by age. Research is needed on the effects of RAR-focused interventions on brain health.