Graveyard work shift takes a toll on the heart

Long-term exposure to night shift work is associated with risks of atrial fibrillation (AF) and coronary heart disease (CHD), a study suggests.

“Night shift work may break the body’s circadian rhythm and affect metabolism and hormone secretion. [T]he effect may be more significant with long-term night shift exposure,” said the researchers.

“After irregular work, disruption of the adaptability to environmental changes, sympathetic nerve excitement, and excessive secretion of norepinephrine enhance myocardial contractility, increase conduction speed and oxygen consumption, and further aggravate the load on the heart,” they continued. Furthermore, night shift work has been tied to increased cardiovascular inflammatory markers owing to the higher degree of oxidative stress damage among those who work nights. [Proc Natl Acad Sci USA 2016;113:E1402-E1411; Ann Occup Environ Med 2016;28:48; Sci Rep 2019;9:4455]

The team evaluated a large cohort of participants without AF (n=283,657) from the UK Biobank who were either in paid employment or self-employed. Of these, 276,009 had no CHD, stroke, and HF at baseline, and 193,819 had genetic data. During a median follow-up of 10.4 years, 5,777 incident AF cases were documented. Most participants (83 percent) were classified as ‘day workers’, while the rest worked nights either on a ‘rarely’, ‘some’, or ‘usual/permanent’ basis (8, 5, and 4 percent, respectively). [Eur Heart J 2021;doi:10.1093/eurheartj/ehab505]

“In this large-scale cohort with a 10-year follow-up time, we [had several findings … First,] we found that current night shift work was associated with increased risks of AF … From ‘day workers’, ‘shift but never/rarely night shifts’, and ‘some night shifts’ to ‘usual/permanent night shifts’, there was a significant increasing trend in incident AF and CHD risk,” said the researchers.

Compared with day workers, the increasing categories of night shifts were associated with a higher risk of incident AF, with those on usual/permanent night shifts having the greatest risk after adjusting for multiple* covariates (hazard ratio [HR], 1.12; p_trend=0.013).

Longer durations of night shift work exposure were also significantly associated with higher AF risk, particularly among those exposed to night shift work for ≥10 years (HR, 1.18; p_trend=0.085), more so among participants with incident AF ≥1 year from baseline who worked nights for ≥10 years (HR, 1.21). Frequency-wise, participants who worked 3–8 nights/month on average had a higher AF risk compared with those who never worked nights after adjusting for multiple covariates (HR, 1.22).

A higher CHD risk was also seen among those working usual/permanent current night shifts (HR, 1.22; p_trend<0.001) and those exposed to night shift work for ≥10 years (HR, 1.37; p_trend<0.001) and 3–8 nights/month (HR, 1.35; p_trend=0.002). The associations between night shift work and stroke and HF across the different groups were nonsignificant.

The role of genetics

Compared with individuals with low genetic risk, higher AF risks were found among those with intermediate and high GRS** (HRs, 1.59 and 3.39, respectively). Nonetheless, genetic predisposition to AF did not seem to influence these positive associations, given the lack of interaction between GRS categories and current night shift status (p_interaction=0.70), lifetime duration (p_interaction=0.79), or frequency of night shift work (p_interaction=0.96).

“This means that if the associations are causal, reducing night shift work frequency and duration might benefit AF risk regardless of the genetic predisposition to AF,” the researchers explained.

Public health implications

Taken together, the findings show that current and lifetime exposure to night shifts were significantly associated with AF risk, regardless of genetic risk. Night shift exposure also increased CHD, but not stroke or HF, risk. “Our findings [also] have public health implications for primary AF prevention. In fact, primary AF prevention seems to be not well-mentioned in guidelines, indicating evidence lacking in this field,” the researchers pointed out.

The current findings also align with recommendations advocating work schedule adjustments to improve arrhythmia. “Because changes in shift work schedules could potentially reduce such risk, and [given the] increasing prevalence of night shift work exposure [nowadays], it is important to consider and further explore the relationship between shift schedules and AF risk, even though the risk would be relatively small,” said the researchers.

“Whether decreasing night shift work frequency and duration might represent another avenue to improve heart health during working life and beyond warrants further study,” the researchers concluded.