According to a study presented at the ESC Preventive Cardiology 2021, an online scientific congress of the European Society of Cardiology associated greater cardiovascular risk (the risk of heart disease) with Working hours that deviate from an individual’s natural body clock.
“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,” said study author Dr Sara Gamboa Madeira of the University of Lisbon, Portugal.
It is estimated that 20% of European employees work unusual hours or shifts, and a growing body of scientific evidence links this to adverse cardiovascular outcomes. Several explanations have been put forward, including sleep disturbances and unhealthy behaviours. This study focuses on the role of circadian misalignment, i.e the difference between an individual’s “social clock” (e.g. working hours) and “biological clock”.
The study involved 301 workers, all of whom performed manual picking at the distribution warehouse of a retail company in Portugal. The staff always work in the morning (6:00 – 15:00), late in the evening (15:00 – midnight) or at night (9:00 – 06:00). Participants filled out a questionnaire on socio-demographic factors (age, gender, education), occupational factors (working time, seniority), and lifestyle factors, and measured their blood pressure and cholesterol.
The Munich Chronotype Questionnaire was used to determine sleep duration and to estimate each individual’s internal biological clock (also called chronotype). It has also been used to measure the amount of circadian mismatch (i.e., the mismatch between individual biological clocks and working hours) – so-called ‘social jet lag’. Participants were divided into three groups based on social jet lag hours: 2 hours or less, 2-4 hours, 4 hours or more.
The researchers used the European relative risk SCORE chart which incorporates smoking, blood pressure and cholesterol to calculate relative cardiovascular risk. Relative risk ranges from 1 (non-smoker with healthy blood pressure and cholesterol) to 12 (smoker with very high blood pressure and cholesterol). In this study, a relative risk of 3 or more was considered “high cardiovascular risk”. The researchers then investigated the association between social jetlag and high cardiovascular risk.
The average age of participants was 33 years and 56% were men. Just over half (51%) were smokers, 49% had high cholesterol, and 10% had hypertension. One in five (20%) were classified as high cardiovascular risk. Some 40% had a short sleep duration on workdays (6 hours or less). The average social jetlag was nearly 2 hours. In most workers (59%), social jetlag was 2 hours or less, while for 33% of staff it was 2–4 hours, and in 8% it was 4 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% for each additional hour of social jetlag, even after adjusting for sociodemographic, occupational, lifestyle, and sleep characteristics and body mass index.
Dr Gamboa Madeira said: “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. Longitudinal studies are needed to investigate whether late chronotypes cope better with late/night shifts and earlier chronotypes to early morning schedules, both psychologically and physiologically.”
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