Wrist actigraphy has been used to assess sleep in older adult populations for nearly half a century. Over the years, the continuous raw activity data derived from actigraphy has been used for the characterization of factors beyond sleep/wake such as physical activity patterns and circadian rhythms. Behavioral activity rhythms (BAR) are useful to describe individual daily behavioral patterns beyond sleep and wake, which represent important and meaningful clinical outcomes. This paper reviews common rhythmometric approaches and summarizes the available data from the use of these different approaches in older adult populations. We further consider a new approach developed in our laboratory designed to provide graphical characterization of BAR for the observed behavioral phenomenon of activity patterns across time. We illustrate the application of this new approach using actigraphy data collected from a well-characterized sample of older adults (age 60+) with osteoarthritis (OA) pain and insomnia. Generalized additive models (GAM) were implemented to fit smoothed nonlinear curves to log-transformed aggregated actigraphy-derived activity measurements. This approach demonstrated an overall strong model fit (R² = 0.82, SD = 0.09) and was able to provide meaningful outcome measures allowing for graphical and parameterized characterization of the observed activity patterns within this sample.

Hyperarousal is a 24-h state of elevated cognitive and physiological activation, and is a core feature of insomnia. The extent to which sleep quality is affected by stressful events—so-called sleep reactivity—is a vulnerability factor for developing insomnia. Given the increasing prevalence of insomnia with age, we aimed to investigate how hyperarousal and sleep reactivity were related to insomnia severity in different adult age groups. Data were derived from a large cohort study investigating the natural history of insomnia in a population-based sample (n = 1693). Baseline data of the Arousal Predisposition Scale (APS) and Ford Insomnia Response to Stress Test (FIRST) were examined across age and sleep/insomnia subgroups: 25–35 (n = 448), 35–45 (n = 528), and 45–55 year olds (n = 717); good sleepers (n = 931), individuals with insomnia symptoms (n = 450), and individuals with an insomnia syndrome (n = 312). Results from factorial analyses of variance (ANOVA) showed that APS scores decreased with increasing age, but increased with more severe sleep problems. FIRST scores were not significantly different across age groups, but showed the same strong increase as a function of sleep problem severity. The findings indicate that though arousal predisposition and sleep reactivity increase with more severe sleep problems, only arousal decreases with age. How arousing events affect an individual during daytime thus decreases with age, but how this arousal disrupts sleep is equivalent across different adult age groups. The main implication of these findings is that treatment of insomnia could be adapted for different age groups and take into consideration vulnerability factors such as hyperarousal and stress reactivity.

Amyloid beta (Aβ) plaque burden, as measured by positron emission tomography (PET), is increasingly being used as a biomarker for Alzheimer's disease (AD) as well as a screening or monitoring tool for clinical trials with amyloid-lowering drugs. However, PET imaging is expensive, invasive and not widely available for all patients, necessitating alternative means to assess brain Aβ accumulation. In this study, we measured levels of Aβ42, Aβ40 and Aβ38 in saliva samples from cognitively unimpaired older adults (n=93; 61.7 % female; mean age = 70.1 ± 6.6 years) using the Mesoscale Discovery platform, carefully considering preanalytical variables, including timing of sample collection, blood contamination and sample concentration. We next determined the relationships between Aβ peptide levels and Aβ plaque burden within the brain, determined using 18F-florbetapir (FBP) PET. We found that salivary levels of Aβ38 and Aβ42, but not Aβ40 nor the Aβ42/Aβ40, were significantly positively correlated with the global mean FBP standardized uptake value ratio (SUVR), before and after adjusting for age, sex and time of day of saliva sample collection (r=0.523/0.544, p=0.001/0.002 and r=0.316/0.32, p=0.031/0.044, for Aβ38 and Aβ42, respectively). Similar results were observed when Aβ values were analyzed as a ratio to the total protein levels in each sample and when tested in saliva samples that were collected during a restricted morning time window. Using composite regions which represent cortical regions vulnerable to Aβ accumulation in early, intermediate, and late stages of AD, we found that Aβ38 showed the most robust correlation with FBP SUVRs from early-accumulating brain regions (r=0.510; p<0.001). In contrast to the observed effects in saliva, plasma levels of Aβ42 measured from a subset of the participants showed a significant negative correlation to mean FBP SUVR. Using logistic regression analysis to determine whether any salivary Aβ species could predict brain Aβ burden, we found that salivary levels of Aβ38 in combination with age, sex, sample timing and APOE genotype could predict Aβ-PET positivity with an area under the curve = 0.950 (95 % confidence interval, 0.876-1.0; p<0.0001). Our findings suggest that salivary Aβ38 and/or Aβ42 could have relevance as a non-invasive, and more widely applicable biomarker, for utility in clinical studies on AD.

In healthy adolescents, better cardiorespiratory fitness, as assessed by the gold standard measure [laboratory-based assessment of peak oxygen consumption (V̇o₂)], was associated with earlier circadian timing of sleep/wake patterns, rest-activity rhythms and chronotype, and less social jetlag. These findings highlight the close interrelationships between fitness and rhythms and raise the possibility that maintaining higher cardiorespiratory fitness levels alongside earlier sleep/wake schedule and activity rhythms may be important behavioral intervention targets to promote health in adolescents.