Studies link shift work to cancer

Studies link shift work to cancer
Having a work environment that has workers on frequent, irregular rotation shift schedule may be causing them more than just sleepless nights. 

A World Health Organization study on the carcinogenic effects of shift work showed there’s “modestly increased” risk of breast cancer among long-term employees who performed night shift work.
According to Dr. Joan Saary, assistant professor in the Division of Occupational Medicine at the University of Toronto and a physician at St. Michael’s Hospital, shift work results in the disruption of the human body’s circadian rhythm — essentially, the biological clock.

The WHO study noted that while there is limited evidence in humans for the carcinogenicity of shift work that involves night work, there is sufficient evidence in experimental animals for the carcinogenicity of exposure to light during nighttime.

These results led researchers to conclude that shift work that involves circadian disruption is “probably carcinogenic” to humans, Saary said.

Saary, who is also an occupational medicine consultant for the Canadian Forces Environmental Medicine Establishment, presented at a recent Centre for Health and Safety Innovation Research Exchange Series hosted by the Workplace Safety and Prevention Services.

Her presentation focused on the importance of sleep, as well as the challenges and health effects of shift work.

“Sleep must be continuous and uninterrupted to be restorative,” said Saary. “Seemingly minor irritations like noise, temperature and light can keep us from reaching critical sleep stages, leading to inadequate replenishment of energy stores resulting in lack of energy for physical and mental tasks, and compromised safety-critical performance.”

Human biological clocks use light as a marker, Saary explained. Melatonin is the hormone that regulates circadian rhythms and it’s produced in the absence of light. Workers on night shift are exposed to light and therefore don’t usually get their dose of melatonin when it’s supposed to kick in.

During the day, when night shift workers are supposed to sleep, disruptions and other external factors may prevent the worker from having uninterrupted sleep, which is critical for producing high levels of melatonin.

Among the many different patterns of shift work, those that include night work are most disruptive to the circadian system, Saary said.

“DNA repair is part of the process that happens at night. If it doesn’t get repaired, that’s when things start to go awry,” she said.

Experimental studies have also shown that reducing melatonin levels at night increases the incidence or growth of tumors.

In Canada, about 24 per cent of full-time workers perform shift work. Manufacturing, health care and retail are industries with the highest number of shift workers.

Shift work is also linked to other types of health problems including, diabetes, cardiovascular disease, sleep disorders, gastrointestinal problems, respiratory, reproductive risks, as well as psychological issues.

Irregular shift work patterns may also affect a workers’ performance, alertness on the job and even pose safety risks that can be caused by fatigue-related poor judgment, Saary said.

All is not lost for shift workers, however. According to Saary, the trick is in optimizing adaptation to a shifting work schedule.

Despite the negative effects, about 70 per cent of workers generally withstand and stay on shift work, but they do manifest different levels of intolerance at different times.

In other words, said Saary, “they put up with it…they don’t necessarily enjoy it but they somehow manage.”

Maintaining a regular sleep/wake schedule even on days off can help workers adapt more effectively to shift work. Increasing sleep quantities at bedtime and on weekends — allowing for continuous, uninterrupted sleep to last at least seven to eight hours — can also help mitigate the negative effects of shift work, Saary said.

Workers can also implement sleep strategies, such as making the room dark and avoiding full meals before bedtime.

Good dietary habits can also go a long way: drink plenty of fluids; avoid fatty, salty and junk food; minimize caffeine intake; and for night shifts, eat lightly through the shift.

Employers could also help by designing work schedules that will allow workers to better adapt to shift work. Saary outlined some of them:
  • Avoid early morning shift starts that begin between 4AM and 6AM
  • Allow adequate time off between shifts
  • Provide one or more rest days between shifts
  • Provide a predictable work schedule
  • If possible, limit weekend work
  • Allow individual or employees input to scheduling