Irregular 24-hour activity rhythms and the metabolic syndrome in older adults

Circadian rhythms - near 24 h intrinsic biological rhythms - modulate many aspects of human physiology and hence disruption of circadian rhythms may have an important impact on human health. Experimental work supports a potential link between irregular circadian rhythms and several key risk factors for cardiovascular disease including hypertension, obesity, diabetes and dyslipidemia, collectively termed the metabolic syndrome. While several epidemiological studies have demonstrated an association between shift-work and the components of the metabolic syndrome in working-age adults, there is a relative paucity of data concerning the impact of non-occupational circadian irregularity in older women and men. To address this question, we studied 7 days of actigraphic data from 1137 older woman and men participating in the Rush Memory and Aging Project, a community-based cohort study of the chronic conditions of aging. The regularity of activity rhythms was quantified using the nonparametric interdaily stability metric, and was related to the metabolic syndrome and its components obesity, hypertension, diabetes and dyslipidemia. More regular activity rhythms were associated with a lower odds of having the metabolic syndrome (OR = 0.69, 95% CI = 0.60-0.80, p = 5.8 x 10(-7)), being obese (OR = 0.73, 95% CI = 0.63-0.85, p = 2.5 x 10(-5)), diabetic (OR = 0.76, 95% CI = 0.65-0.90, p = 9.3 x 10(-4)), hypertensive (OR = 0.78, 95% CI = 0.66-0.91, p = 2.0 x 10(-3)) or dyslipidemic (OR = 0.82, 95% CI = 0.72-0.92, p = 1.2 x 10(-3)). These associations were independent of differences in objectively measured total daily physical activity or rest, and were not accounted for by prevalent coronary artery disease, stroke or peripheral artery disease. Moreover, more regular activity rhythms were associated with lower odds of having cardiovascular disease (OR = 0.83; 95% CI = 0.73-0.95, p = 5.7 x 10(-3)), an effect that was statistically mediated by the metabolic syndrome. We conclude that irregular activity rhythms are associated with several key components of the metabolic syndrome in older community-dwelling adults, and that the metabolic syndrome statistically partially mediates the association between activity rhythms and prevalent cardiovascular disease. Although additional longitudinal and experimental studies are needed to conclusively delineate the causal relationships underlying these associations, these findings are consistent with preclinical data, and add further support for investigations of the irregularity of activity rhythms as a potential therapeutic target to decrease the burden of cardiovascular disease in older adults.