Abstract
Several studies implicate circadian rhythm disturbances in Alzheimer's disease. However, very little is known about how circadian rhythms are associated with Alzheimer's pathological biomarkers in older adults at early stages of the disease, and how these relationships map onto cognition. This cross-sectional study used 24-h accelerometry data to investigate the relationships between circadian rhythms, amyloid-β (Aβ), tau, and cognition in 68 older adults with objective early cognitive impairment. Participants wore GENEActiv accelerometers for ∼1 month (mean = 31.8 days). Circadian rhythms measures were quantified from accelerometer data and included acrotime (average time of day of peak activity) and intradaily variability (IV) (average circadian rhythm fragmentation within a day). Aβ was measured as a composite, and tau (n = 67) was measured in Braak staging regions of interest I/II and III/IV using positron emission tomography. The cognitive domains used were verbal memory (California Verbal Learning Test short delay free recall) and attention/processing speed (Digit Symbol Substitution Test). Multivariable linear regression models were conducted to test for associations between circadian rhythms and the outcome variables of Aβ, tau, and cognition. The moderating effects of age, sex, and apolipoprotein E4 (APOE4) carrier status were assessed in these associations. To investigate mechanistic pathways through which circadian rhythms may impact cognition, exploratory mediation analyses were conducted post hoc. Models were adjusted for age, sex, APOE4 carrier status, and years of education. The study included 68 older adults (mean age = 66.8 years, age range = 55-80 years, 63.2% female, 26.5% APOE4 carriers). Earlier acrotime was associated with higher Aβ and tau, the former of which was stronger in APOE4 carriers relative to non-carriers. Higher IV was related to higher tau in Braak regions III/IV. Age and sex modified the association between IV and tau, in which the relationships strengthened with increasing age and disproportionately affected men. Earlier acrotime was associated with worse verbal memory, but later acrotime was associated with worse attention/processing speed. Tau in Braak regions I-IV mediated the relationship between acrotime and verbal memory. The insights from this study revealed that circadian rhythms were associated with Aβ, tau, and cognition in older adults with objective early cognitive impairment. We provide novel evidence for tau as a biological mediator in the relationship between circadian timing and cognition. This work identified circadian rhythms as a promising point of intervention to reduce Alzheimer's disease risk and potentially mitigate pathological progression and cognitive decline.