STAMI has studied risk factors for cardiovascular disease in industrial shift workers rotating between day, afternoon/evening and night shifts and compared them to industrial day workers. The project started in 2018 and will continue until 2024.

The project has followed industrial workers from two different industrial plants in Østfold county. One group worked rotating shifts including night shifts, the other worked day shifts.

– Our study shows that working rotating shifts which includes night work, compared to day work, is associated with arterial stiffening – a known risk factor for future cardiovascular disease. We also found some support for an association between rotating shift work and systemic inflammation, says senior physician, research professor, and project manager Marit Skogstad.

Arterial stiffening is a thickening and stiffening of the arterial wall, and one of the earliest manifestations of cardiovascular disease, independent of high blood pressure.

New methods

Several previous studies indicate an association between cumulative years of rotating shift work and the increase in cardiovascular risk factors. Assumed underlying mechanisms include high blood pressure, increased inflammation, obesity, diabetes, and low levels of physical activity.

This STAMI-project applies new and advanced methods to reveal effects on the cardiovascular system. Earlier studies have primarily been based on registry data. In this study, however, the researchers have collected biological samples from employees close to or in their regular working environment.

The aim is to assess changes in cardiovascular risk factors during a six-year follow-up period.

– We are looking for early markers for cardiovascular disease. This is done by examining the carotid artery, measuring blood pressure, arterial stiffness by the means of pulse wave velocity, maximal oxygen uptake as a measure of cardiorespiratory fitness, glycosylated haemoglobin, lipids such as cholesterol, and inflammatory markers in the blood, says Skogstad.

Humans are active during daytime

The reason why night shifts may increase the risk for cardiovascular disease could, simply put, be that humans are active during daytime, the researcher says. Thus, staying awake during the night will interfere with our circadian rhythm (our internal biological clocks). This disturbance will affect almost every cell, organ, and bodily function.

– Disrupted circadian rhythms may explain the elevated risk of cardiovascular disease. Sleep deficits are associated with more low-grade inflammation in the body. Our studies suggest that shift work including night work also may be associated with increased thickening of the carotid artery, explains Skogstad.

When examining the development over a three-year period, the researchers found more arterial stiffening in the shift workers than in the day workers. The shift workers also had higher levels of certain inflammation markers in their blood than the day workers had, Skogstad says.

Positive effects of sleep and physical activity

A sub-study examined the effects of changing the shift schedule by reducing the number of consecutive night shifts from seven to four. During a twelve-weeks’ period, the total number of night shifts was reduced from 17 to 14 in one of the factories. Surprisingly, the researchers found no difference in arterial stiffness when comparing the measurements from these workers to those from the other factory the last year of follow-up.

In another sub-study, Skogstad and colleagues found results that indicated a decrease in cardiovascular risk factors among workers who participated in a high-intensity exercise program.

Offers a partial explanation

Skogstad says the findings in this project offers a partial explanation to why shift workers have a higher risk of developing cardiovascular diseases than their peers in the same industry who only work day shifts.

However, there are also other factors that may have an influence on early manifestations of cardiovascular disease. For instance, in industrial plants the workers will be exposed to dust.

− Dust assessment is important because long-duration and high-level exposure to respirable particulate matter is regarded as a potential threat to vascular health, Skogstad explains.

The industrial plants examined in this project produced insulation material, which meant that the workers were exposed to particulate matter. The researchers did dust measurements in both plants, including size and number of particles. They concluded that the level of dust can be compared to the level in regular city air.

Important knowledge

The researcher emphasizes that these findings don’t mean working rotating shifts invariably will lead to cardiovascular disease.

– Even with this new knowledge, it’s important to remember that in industry there are other possible risk factors for cardiovascular disease as well. We cannot rule out a connection between cardiovascular disease and other kinds of adverse exposure in the working environment. Self-reported physical activity may also be somewhat biased, Skogstad points out.

− However, there is no doubt that this project has yielded new and interesting findings that can clarify which exposures in the working environment could affect workers’ health. The project has given us new knowledge of the association between long-time exposure to rotating shift work including working at night, and future risk for cardiovascular disease, Skogstad concludes.