Can yesterday's smoking research inform today's shiftwork research? Epistemological consequences for exposures and doses due to circadian disruption at and off work

The Role of the Circadian Rhythm in Dyslipidaemia and Vascular Inflammation Leading to Atherosclerosis

Cardiovascular diseases (CVD) are among the leading causes of death worldwide. Many lines of evidence suggest that the disturbances in circadian rhythm are responsible for the development of CVDs; however, circadian misalignment is not yet a treatable trait in clinical practice. The circadian rhythm is controlled by the central clock located in the suprachiasmatic nucleus and clock genes (molecular clock) located in all cells. Dyslipidaemia and vascular inflammation are two hallmarks of atherosclerosis and numerous experimental studies conclude that they are under direct influence by both central and molecular clocks. This review will summarise the results of experimental studies on lipid metabolism, vascular inflammation and circadian rhythm, and translate them into the pathophysiology of atherosclerosis and cardiovascular disease. We discuss the effect of time-respected administration of medications in cardiovascular medicine. We review the evidence on the effect of bright light and melatonin on cardiovascular health, lipid metabolism and vascular inflammation. Finally, we suggest an agenda for future research and recommend on clinical practice.

Role of Melatonin in Cancer: Effect on Clock Genes

The circadian clock is a regulatory system, with a periodicity of approximately 24 h, that generates rhythmic changes in many physiological processes. Increasing evidence links chronodisruption with aberrant functionality in clock gene expression, resulting in multiple diseases, including cancer. In this context, tumor cells have an altered circadian machinery compared to normal cells, which deregulates the cell cycle, repair mechanisms, energy metabolism and other processes. Melatonin is the main hormone produced by the pineal gland, whose production and secretion oscillates in accordance with the light:dark cycle. In addition, melatonin regulates the expression of clock genes, including those in cancer cells, which could play a key role in the numerous oncostatic effects of this hormone. This review aims to describe and clarify the role of clock genes in cancer, as well as the possible mechanisms of the action of melatonin through which it regulates the expression of the tumor's circadian machinery, in order to propose future anti-neoplastic clinical treatments.

Clocks, cancer, and chronochemotherapy

The circadian clock coordinates daily rhythmicity of biochemical, physiologic, and behavioral functions in humans. Gene expression, cell division, and DNA repair are modulated by the clock, which gives rise to the hypothesis that clock dysfunction may predispose individuals to cancer. Although the results of many epidemiologic and animal studies are consistent with there being a role for the clock in the genesis and progression of tumors, available data are insufficient to conclude that clock disruption is generally carcinogenic. Similarly, studies have suggested a circadian time-dependent efficacy of chemotherapy, but clinical trials of chronochemotherapy have not demonstrated improved outcomes compared with conventional regimens. Future hypothesis-driven and discovery-oriented research should focus on specific interactions between clock components and carcinogenic mechanisms to realize the full clinical potential of the relationship between clocks and cancer.

Sleep Irregularity and Risk of Cardiovascular Events: The Multi-Ethnic Study of Atherosclerosis

BACKGROUND

The cardiovascular system exhibits strong circadian rhythms to maintain normal functioning. Irregular sleep schedules, characterized by high day-to-day variability in sleep duration or timing, represent possibly milder but much more common and chronic disruption of circadian rhythms in the general population than shift work.

OBJECTIVES

This study aimed to prospectively examine the association between sleep regularity and risk of cardiovascular disease (CVD).

METHODS

In MESA (Multi-Ethnic Study of Atherosclerosis), 1,992 participants free of CVD completed 7-day wrist actigraphy for sleep assessment from 2010 to 2013 and were prospectively followed through 2016. The study assessed sleep regularity using the SD of actigraphy-measured sleep duration and sleep-onset timing across 7 days. Incident CVD included nonfatal and fatal cardiovascular events. A Cox proportional hazards model was used to estimate hazard ratios (HRs) for incident CVD according to SD of sleep duration and timing, adjusted for traditional CVD risk factors (including CVD biomarkers) and other sleep-related factors (including average sleep duration).

RESULTS

During a median follow-up of 4.9 years, 111 participants developed CVD events. The multivariable-adjusted HRs (95% confidence intervals) for CVD across categories of sleep duration SD were 1.00 (reference) for ≤60 min, 1.09 (0.62 to 1.92) for 61 to 90 min, 1.59 (0.91 to 2.76) for 91 to 120 min, and 2.14 (1.24 to 3.68) for >120 min (p trend = 0.002). Similarly, compared with participants with a sleep timing SD ≤30 min, the HRs (95% confidence intervals) for CVD were 1.16 (0.64 to 2.13) for 31 to 60 min, 1.52 (0.81 to 2.88) for 61 to 90 min, and 2.11 (1.13 to 3.91) for >90 min (p trend = 0.002). Exclusion of current shift workers yielded similar results.

CONCLUSIONS

Irregular sleep duration and timing may be novel risk factors for CVD, independent of traditional CVD risk factors and sleep quantity and/or quality.

The riddle of shiftwork and disturbed chronobiology: a case study of landmark smoking data demonstrates fallacies of not considering the ubiquity of an exposure

BACKGROUND

Failing to integrate all sources of a ubiquitous hazard candidate may explain inconsistent and/or null, and overall misleading, results in epidemiological studies such as those related to shift-work.

METHODS

We explore this rationale on the assumption that Doll and Hill had confined their 1950 landmark study to smoking at workplaces alone. We assess how non-differential, or how differential, underestimation of exposure could have biased computed risks.

RESULTS

Systematically unappreciated exposures at play could have led to substantial information bias. Beyond affecting the magnitude of risks, not even the direction of risk distortion would have been predictable.

CONCLUSIONS

Disturbed chronobiology research should consider cumulative doses from all walks of life. This is a conditio sine qua non to avoid potentially biased and uninterpretable risk estimates when assessing effects of a ubiquitous hazard candidate.

IARC 2019: "Night shift work" is probably carcinogenic: What about disturbed chronobiology in all walks of life?

In June of 2019, a working group convened by the International Agency for Research on Cancer [IARC] concluded that “night shift work” is probably carcinogenic to humans (a Group 2A carcinogen). This was based on sufficient evidence of cancer and strong mechanistic evidence in experimental animals and limited evidence from human epidemiological studies. The biological basis from experimental work is clear and compelling: Disturbed chronobiology such as due to alterations in the light-dark schedule which shift-workers experience is associated with carcinogenicity. But is it correct to assume in epidemiological studies that “night shift work” provides the same dose of disturbed chronobiology to all night workers and that disturbed chronobiology from activities outside of work does not count? Both chronobiological theory and supporting evidence suggest that much-needed future epidemiology should address these questions and should consider disturbed chronobiology in all walks of life.