Working night shifts for extended periods of time increase mortality related to the development of lung cancer and cardiovascular disease (CVD), particularly among women, according to study recently published at the American Journal of Preventive Medicine. It was already known that working rotating night shifts was detrimental to health, however, researchers were now able to determine the extent of its effects for the first time.
Women who work in rotating night shifts during more than five years registered a modest higher mortality due to “all-cause” and CVD, while when the period is higher than 15 years, the scientists found a modest increase in mortality related to lung cancer. The reason may have to do both with sleep and the circadian system, which are two crucial elements for cardiovascular health and anti-tumor activity.
The research team, composed of international scientists, has found substantial biological data to conclude that working night shifts increases the possibility of suffering from either lung cancer or CVD, and that it contributes to a higher rate of mortality related to the diseases. The researchers analyzed information from approximately 75,000 registered U.S. nurses from the Nurses’ Health Study (NHS) on a follow-up study that lasted 22 years.
When comparing all-cause, CVD, and cancer mortality, the scientists demonstrated that five years is enough to increase both all-cause and CVD mortality. Lung cancer, on the other hand, was only at higher risk for professionals working night shifts for more than 15 years, and they reported a 25% higher risk. In addition, lung cancer was the only type of cancer to register higher mortality related to night shift working.
This study “is one of the largest prospective cohort studies worldwide with a high proportion of rotating night shift workers and long follow-up time,” explained the Associate Professor of Medicine at Harvard Medical School, and Associate Epidemiologist, at the Department of Medicine of Brigham and Women’s Hospital, Boston, Eva S. Schernhammer, MD, DrPH. “A single occupation (nursing) provides more internal validity than a range of different occupational groups, where the association between shift work and disease outcomes could be confounded by occupational differences.”
The study enrolled 121,700 U.S. female nurses between the ages of 30 and 55 years old in 1976 who responded to questionnaires twice a year ever since. Then, the researchers collected information on night shifts in 1988, when 85,197 women participated, and a group of nurses were excluded for suffering from pre-existing CVD or other than non-melanoma skin cancer, while another 74,862 were included. During the questionnaires, the nurses were asked how many years they worked rotating shifts, defined as working at least three nights per month in addition to days or evenings in that month.
“These results add to prior evidence of a potentially detrimental relation of rotating night shift work and health and longevity. To derive practical implications for shift workers and their health, the role of duration and intensity of rotating night shift work and the interplay of shift schedules with individual traits (e.g., chronotype) warrant further exploration,” added Schernhammer.