Abstract
Purpose of ReviewThis review aims to explore how circadian rhythms influence disease susceptibility and potentially modify the effect of environmental exposures. We aimed to identify biomarkers commonly used in environmental health research that have also been the subject of chronobiology studies, in order to review circadian rhythms of relevance to environmental health and determine if time-of-day is an important factor to consider in environmental health studies. Moreover, we discuss opportunities for studying how environmental exposures may interact with circadian rhythms to structure disease pathology and etiology.Recent FindingsIn recent years, the study of circadian rhythms in mammals has flourished. Animal models revealed that all body tissues have circadian rhythms. In humans, circadian rhythms were also shown to exist at multiple levels of organization: molecular, cellular, and physiological processes, including responding to oxidative stress, cell trafficking, and sex hormone production, respectively. Together, these rhythms are an essential component of human physiology and can shape an individual’s susceptibility and response to disease.SummaryCircadian rhythms are relatively unexplored in environmental health research. However, circadian clocks control many physiological and behavioral processes that impact exposure pathways and disease systems. We believe this review will motivate new studies of (i) the impact of exposures on circadian rhythms, (ii) how circadian rhythms modify the effect of environmental exposures, and (iii) how time-of-day impacts our ability to observe the body’s response to exposure.
Highlights
Circadian rhythms exist in species throughout the tree of life, from single-cell organisms to humans
We discuss circadian impacts on the pathophysiology and/or treatment of asthma, heart disease, and breast cancer because these are diseases for which both chronobiology and environmental health research exist to date, and we explore how circadian rhythms influence biological targets of environmental exposures and biomarkers that are commonly used in the study of environmental exposures (Fig. 1)
We describe how circadian rhythms in underlying biological processes can shape the overall presentation of disease and how circadian disruption due to occupational environmental exposures may elevate risk of disease
Summary
Circadian rhythms exist in species throughout the tree of life, from single-cell organisms (e.g., cyanobacteria and Trypanosomes) to humans. Circadian disruption occurs due to a lack of synchronization between body clocks and the day-night cycle and can cause misalignment among the various circadian clocks present throughout the body [8]. Well-known causes of circadian disruption/misalignment include various occupational and household environmental factors such as shift work [9], jetlag [10], and exposure to artificial light-at-night [11]. Disease risk associated with circadian disruption led the American Medical Association in 2012 to adopt a policy statement concluding that “The natural 24-hour cycle of light and dark helps maintain precise alignment of circadian biological rhythms,...Pervasive use of nighttime lighting disrupts these endogenous processes and creates potentially harmful health effects and/or hazardous situations with varying degrees of harm” [12, 15], and, in 2007, the World Health Organization’s International Agency for Research on Cancer (IARC) classified shift work that involves circadian disruption as a probable human carcinogen [16]. We discuss ways in which studies of environmental exposures and environmental determinants of health can deploy circadian and time-dependent sampling to study how circadian rhythms may act as a modifier of disease
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