The European Institution of Occupational Safety and Health (IOSH) has issued a new research paper on the relation between working night shifts and cancer. While the study looks at multiple cancers—prostate, ovarian, colorectal, endometrial, and non-Hodgkin’s lymphoma—the primary focus is on breast cancer in women.
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The paper notes that since the possible connection between night work or shiftwork and breast cancer came to the attention of scientists more than 15 years ago, increasingly well-conducted epidemiological studies have delved into the connection between cancer and chronodisruption, the interruption of the body’s natural circadian rhythm. While the paper does not dismiss the relationship, it suggests that the connection, if it exists at all, is not as strong as once believed.
“While there is some evidence of an exposure-response relationship between the occurrence of breast cancer and the length of time working night shifts, it is probably not sufficiently strong to suggest limiting the length of time that individuals continue to work night shifts,” concludes the IOSH.
The IOSH researchers identified 36 scientific papers that examined the epidemiological evidence relating to shiftwork and cancer, and a further 50 were identified that looked at the potential process of causation for specific cancers.
Melatonin Deprivation
The paper identifies the suppression of melatonin as a possible cause of cancer. Melatonin is a hormone produced by the pineal gland in the brain and is associated with control of sleep/wake cycles (circadian rhythms). Melatonin is known to have anticarcinogenic properties, but its production is reduced in humans when they are exposed to light.
Increased cancer risk could also be due to the known association between obesity and breast cancer since it is known that shiftworkers may have a poorer diet than dayworkers and may be more likely to be obese or have other lifestyle factors associated with cancer.
Additional Work Needed
The researchers reviewed 15 papers that examined the occurrence of breast cancer in women working night shifts. Overall, the majority of studies showed an increased risk of breast cancer in relation to shiftwork and an increased risk when shifts had been worked for longer periods.
“However, many of the studies had limitations because of the lack of clear descriptions of the shift systems used,” added the IOSH. “The review also examined more recent studies of breast cancer from night work where other risk factors were better controlled. In these studies the association between shift work and breast cancer was still apparent, but the magnitude of the relative risks was reduced from around 1.5 to 1.2, that is, from 50 per cent [sic] higher than expected to 20 per cent [sic] higher.”
“Overall, our researchers have concluded that the measured relative risk of breast cancer in well-conducted epidemiological studies of female night workers is lower than originally identified and may not be significantly raised,” added the IOSH. “Further prospective studies with well-defined shift work data and well controlled for potential confounding factors are still required.”
Recommendations for Employers
Still, the risk cannot be dismissed, and the IOSH said employers should consider implementing the following actions:
- Develop a workplace policy for night work that informs workers about the potential cancer risks and sets out prudent strategies to minimize the impact of shiftwork on health.
- Design rotating shift patterns that help prevent circadian disruption, and ensure that the number of consecutive night shifts worked is minimized
- Instigate health promotion initiatives among nightshift workers to improve their diet and help them maintain a healthy body mass. Provide advice on strategies to improve sleep quality.
- Encourage use of dark glasses on the way home after the shift (weather and light levels permitting) to ensure melatonin levels are not reduced. In addition, promote the use blackout blinds in bedrooms to try to improve daytime sleep quality; and encourage nightshift workers to attend appropriate health screening appointments.
The IOSH report is available at https://www.iosh.co.uk/shiftworkreview.