Timing and intraindividual variability of daytime napping and Alzheimer’s disease in older adults

Introduction

Daytime napping is common among older adults, with a prevalence rate ranging between 20% and 60%¹. Growing evidence suggests that excessive daytime naps may be associated with poorer neurocognitive health. For example, in a sample of 2751 community-dwelling older men, longer nap duration, as measured by actigraphy, was associated with greater cognitive decline and a higher risk for cognitive impairment during a 12-year follow-up period². In addition, our recent study employed objective measurement of daytime naps and found that prolonged nap durations and increased nap frequency were associated with an elevated risk of Alzheimer’s dementia³. However, these existing studies mainly focus on the duration and frequency of naps. In the realm of sleep health, there is a growing body of evidence supporting a multi-dimensional assessment framework to better comprehend the complexities of this vital process⁴. The rationale behind adopting such a multi-dimensional approach is rooted in the understanding that sleep health encompasses more than just the quantity of sleep or the presence/absence of sleep disorders⁴. Additional dimensions, including timing and (ir)regularity, provide valuable insights for a more holistic understanding.

Previous studies have offered important implications regarding the timing and (ir)regularity of daytime naps. For example, naps in the morning may be due to fatigue upon awakening caused by nonrestorative sleep and inflammation⁵. Naps taken in the early afternoon is considered closely aligned with the circadian rhythms, coinciding with the post-lunch dips in alertness and body temperature that also occur at nocturnal bedtime⁶. Additionally, afternoon naps tend to be rich in slow-wave sleep⁷, which has been associated with the clearance of amyloid beta (Aβ) plaques and slower cognitive decline⁸. These prior observations suggest that the timing of naps may play a role in modulating Alzheimer’s disease (AD) pathologies and progression. In terms of (ir)regularity, the intraindividual variability (IIV) in sleep has been associated with a broad spectrum of negative health consequences, including cognition⁹. It remains unclear whether IIV in napping behaviors exhibits similar adverse associations with neurocognitive health.

Therefore, in this work, we seek to utilize the multi-dimensional sleep health framework⁴ to examine the relationships of timing and IIV of daytime napping with...