In an Accompanying Commentary in the same journal, authors added that "owls" (night types) may be less affected by shift work than "larks"(morning types).
The researchers based their findings on 1,101 females with epithelial ovarian cancer (cancer that occurs in the cells lining the ovaries), 389 with borderline disease, who were compared to 1,832 controls (females without ovarian cancer).
The participants were aged between 35 and 74, years. The authors asked them about their working hours, as well as whether they worked night shifts.
According to IARC (International Agency for Research on Cancer), shift work disrupts the body's circadian rhythms and is a cancer-causing agent. Circadian rhythms are mental, physical and behavioral changes that occur during a 24-hour cycle in our bodies, which respond mainly to light and darkness, as well as the environment around us. Circadian rhythms are found in animals, plants and microbes. Our body-clock drives our circadian rhythms.
The authors explained, as background information, that shift work was found to be linked to a greater risk of developing breast cancer in previous studies.
The researchers found that:
- 26.6% of the women with invasive ovarian cancer (1 in every 4) had worked nights
- 32.4% (1 in every 3) among those with borderline disease had worked nights
- 22.5% (1 in every 5) in the control group had worked nights
Among all three groups of women, the stint of night shifts averaged from 2.7 to 3.5 years. Night shift workers were involved in a range of jobs, the most common being food preparation and service, office work, admin support and healthcare.
The authors reported that compared to those who worked normal office hours, working night shifts was linked to:
- A 24% higher risk of developing advanced ovarian cancer
- A 49% greater risk of early stage ovarian cancer
The risks of any type of ovarian cancer were:
- 29% among the larks
- 14% among the owls
- 57% among the larks
- 43% among the owls
Older women (aged 50+ years) were considerably more likely to develop ovarian cancer if they had done night shifts.
The findings in this study are similar to those found in previous studies on breast cancer and shift work, the investigators wrote. However, there appeared to be no growing risk of ovarian cancer the longer a woman continued working night shifts.
Altered melatonin levels at night may impact on cancer riskThe authors suggest that melatonin, a hormone that is normally produced at night and regulates a woman's reproductive hormones, including estrogen, may play a role in increasing ovarian cancer risk among night-time shift workers.
Melatonin, which is not produced in the presence of ambient light, suppresses estrogen levels. Shift workers are exposed to ambient light.
Higher estrogen levels are associated with a higher risk of ovarian cancer.
In an Abstract in the journal, the authors concluded:
"We found evidence suggesting an association between shift work and ovarian cancer. This observation should be followed up in future studies incorporating detailed assessments of diurnal preference (ie, chronotype) in addition to detailed data on shift schedules."
Several studies have linked shift work with cancer risk, but not all
- Breast cancer - after a ruling by a research agency in the United Nations in 2009 that working night shifts probably raises people's risk of developing cancer, the Danish government started paying compensation to women who did shift work and developed breast cancer.
- Prostate cancer - researchers from the University of Occupational and Environmental Health, Japan, found that rotating shift workers have a fourfold greater risk of developing prostate cancer compared to men who do just day shifts or night shifts. Their study, which involved 14,052 workers, was published in the American Journal of Epidemiology (2006 issue).
- A World Health Organization (WHO) study - found that working night shifts raises the risk of developing cancer. The 24 researchers from ten countries identified a higher risk of colon and breast cancers among female shift workers, and a greater risk of prostate cancer among men.
Study leader, Dr. Erhard Haus of HealthPartners Research Foundation, said "Shift work that interferes with regular nighttime sleep disrupts circadian rhythms, our body's natural clock. This impedes biologic function by suppressing the immune system, reducing melatonin production and may damage genes leading to the production of abnormal cells."
- Shift work not linked to cancer risk - a study published in Scandinavian Journal of Work, Environment and Health (September 2007 issue) concluded that working night shift does not seem to raise the risk of developing cancer.
- Light at night (LAN) - researchers from the University of Haifa, Israel, said they found an additional association between "light at night" and cancer. Study leader, Prof. Abraham Haim, said "High power light bulbs contribute more to 'environmental light pollution', which the study has shown is a carcinogenic pollution."
Written by Christian Nordqvist