Metals and Circadian Rhythms

Circadian rhythms describe the behavioral and physiological changes that occur in living organisms in order to attune to a 24 h cycle of day and night. The most striking aspect of circadian function is the sleep–wake cycle; however, many other physiological processes are regulated in 24 h oscillations, including blood pressure, body temperature, appetite, urine production, and the transcription and translation of thousands of circadian-dependent genes. Circadian disruption and sleep disorders are strongly connected to neurodegenerative diseases including Parkinson's disease, Alzheimer's disease, and Huntington's disease, as well as others. Metal exposures have been implicated in neurodegenerative diseases, in some cases involving metals that are essential micronutrients but are toxic at high levels of exposure (such as manganese, copper, and zinc), and in other cases involving metals that have no biological role but are toxic to living systems (such as lead, mercury, and aluminum). In this review, we examine the evidence for circadian and sleep disorders with exposures to these metals and review the literature for possible mechanisms. We suggest that giving the aging population, the prevalence of environmental exposures to metals, and the increasing prevalence of neurodegenerative disease in the aged, more research into the mechanisms of circadian disruption subsequent to metal exposures is warranted.