Workers Health & Safety Centre

Shift work impacts human body clock, damaging heart health: study

Clocking into work when we should be going to bed adds to the risk of suffering a heart attack, stroke and other cardiovascular diseases, a new study suggests.  
The study, published in the May, 2021 issue of the European Journal of Preventive Cardiology, found for each hour a worker’s schedule was out of sync with their natural body clock, their cardiovascular disease (CVD) risk increased by almost 30 per cent.
The human body functions according to a natural sleep-wake/day-night 24 hour cycle referred to as a circadian rhythm. This rhythm helps to maintain many critical body functions ranging from blood pressure and sleep/wake patterns to the normal operation of our immune cells and secretion of hormones. It is guided by environmental cues such as darkness and light and day and night.
“We all have an internal biological clock which ranges from morning types (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,” says study author Dr. Sara Gamboa Madeira of the University of Lisbon, Portugal. “Circadian misalignment occurs when there is a mismatch between what your body wants (e.g. to fall asleep at 10 p.m.) and what your social obligations impose on you (e.g. work until midnight).”

Independent and mounting evidence

Researchers gathered information to quantify the amount of circadian misalignment for each of the workers participating in this study. This discrepancy is known as social jet lag (SJL) and is calculated as the difference in the midpoint of sleep times between work days and non-work days. The average SJL was just under two hours. One in three workers suffered between two and four hours of SJL with eight per cent suffering four hours or more.  
cardiovascular disease (CVD) assessment tool that considers factors including smoking, blood pressure and cholesterol was also used to estimate the overall cardiovascular risk for each participant. More than 20 per cent were found to have a high CVD risk.
The researchers looked at the association between this risk and SJL. Again, the findings suggest the odds of having a “high CVD risk” increases by almost 30 per cent with each additional hour of SJL. This held true even after adjusting for sleep, lifestyle factors, socio-demographics, along with working features. In short, SJL is a risk factor independent of these other factors.
The SJL study adds to the mounting evidence suggesting shiftwork is linked with excess risk of suffering cardiovascular disease.

Wider health implications

According to CAREX Canada, approximately 1.8 million Canadians (and of these, 800,000 Ontarians) work regular night or rotating shift that involves circadian disruption. Industries and occupations relying heavily on shiftwork include health care, social assistance, manufacturing, warehousing, accommodation, food services, transportation, policing, security and the trades.
In addition to the CVD risk discussed above, shiftwork involving circadian disruption may be responsible for 180 to 460 new cases of breast cancer here in Ontario annually. Furthermore, the most recent evaluation by the International Agency for Research on Cancer (2019), re-confirmed classification of night shift work as “probably carcinogenic to humans”. This is based on limited evidence night shift work causes breast, prostate, and colorectal cancer.
Night work is also linked with other health impacts including gastrointestinal disorders, pregnancy complications, mental health issues, work-life balance challenges and elevated risk of workplace incidents resulting in injuries.

Prevention and training 

Eliminating exposure to occupational hazards, including night shift work, should always be the initial consideration when developing and implementing a workplace-specific prevention strategy. Though shift work in many sectors is often unavoidable.
Most efforts focus on administrative controls relating to shift scheduling and rotation. Examples include restricting the number of evening or night shifts, using forward shift rotation (i.e. from day to afternoon to night) and avoiding early starts to day shifts (e.g. not before 6:00am).
The authors of this latest study suggest interventions aimed at reducing SJL, especially for the working schedules of “larks” and “owls”. This might include flexible start times helping them to align their work day with their natural body clock. To this point, research evidence suggests workplace-specific shift system design is more effective if done in a “participatory way” involving workers, worker representatives and supervisors.
For our part, the Workers Health & Safety Centre (WHSC) offers Hours of Work training and a shift work resource line to help workplace parties better understand this common hazard along with workplace strategies aimed at eliminating or mitigating potential health impacts.
We are also a leading provider of Joint Health Safety Committee (JHSC) Certification Training, training for health and safety representatives in smaller workplacesfederal committees and prevention programs and supervisor training.
Some of these programs are offered through our virtual classroom, allowing for the safety of participants and instructors during the COVID-19 crisis. Don’t see what you need? Where participant numbers warrant, we can work with you to coordinate almost any of our programs in a virtual classroom.

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Additional related WHSC resources
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Exposure to night shift work when young raises risk of breast cancer
Night shift work/breast cancer link demands action, says research