Researchers found that working hours that deviate from a person’s natural body clock are associated with a greater risk of cardiovascular disease.

And the more shift work people do, the greater the risk of heart problems.

Study author Dr. Sara Gamboa Madeira, of the University of Lisbon in Portugal, said:

“Our study found that for each hour the work schedule was out of sync with an employee’s body clock, the risk of heart disease got worse.”

At least one in five employees work shifts, and growing scientific evidence links shift work with heart-related health issues.

Several explanations have been proposed, including sleep disruption and unhealthy eating patterns.

The new study focused on the role of circadian misalignment, or “social jetlag” – which is the difference between the “social clock” and the individual “biological clock”.

Dr. Gamboa Madeira said:

“We all have an internal biological clock which ranges from morning types, or larks, who feel alert and productive in the early morning and sleepy in the evening, to late types – owls, for whom the opposite is true, with most of the population falling in between.

“Circadian misalignment occurs when there is a mismatch between what your body wants, for example, to fall asleep at 10 pm, and what your social obligations impose on you, fr example work until midnight.”

The study included 301 blue collar workers, all performing manual picking activity in the distribution warehouses of a retail firm in Portugal.

Staff always worked either early morning (6am-3pm), late evening (3pm-midnight), or night (9pm-6am) shifts. Participants completed a questionnaire on age, sex, education, occupational factors such as work schedule and seniority, and lifestyle factors and had their blood pressure and cholesterol measured.

The questionnaire was used to assess sleep duration, and to estimate each person’s internal biological clock, also called chronotype. It was also used to quantify the amount of circadian misalignment. Participants were divided into three groups according to hours of social jetlag: two hours or less, two to four hours, and four hours or more.

The researchers calculated each participant’s relative cardiovascular risk, taking into account smoking, blood pressure, and cholesterol. They then investigated the association between social jetlag and high cardiovascular risk.

The average age of the participants was 33 years and 56 percent were men. Just over half were smokers, 49 percent had high cholesterol, and 10 percent had high blood pressure. One in five (20 percent) were classified as high cardiovascular risk.

Some 40 per cent had a short sleep duration – six hours or less – on workdays. The average social jetlag was nearly two hours.

In most workers (59 percent), social jetlag was two hours or less, while for a third of staff (33 percent) it was two to four hours, and in around one in 12 (8 percent) it was four hours or more.

A higher level of social jetlag was “significantly associated” with greater odds of being in the high cardiovascular risk group.

The odds of being classified as high cardiovascular risk increased by 31 percent for each additional hour of social jetlag, even after adjusting for other factors such as lifestyle and body mass index.

Dr. Gamboa Madeira added:

“These results add to the growing evidence that circadian misalignment may explain, at least in part, the association found between shift work and detrimental health outcomes.

“The findings suggest that staff with atypical work schedules may need closer monitoring for heart health. Studies are needed to investigate whether late chronotypes cope better with late or night shifts and earlier chronotypes to early morning schedules, both psychologically and physiologically.”

The findings were presented at the European Society of Cardiology’s virtual congress.