Rotating night shifts linked disease outcomes
The term "rotational shiftwork" covers a wide variety of work schedules and implies that shifts rotate or change according to a set schedule according to the Canadian Center for Occupational Health and Safety. These shifts can be either continuous, running 24 hours per day, 7 days per week, or semi-continuous, running 2 or 3 shifts per day with or without weekends. Workers take turns working on all shifts that are part of a particular system. Shiftwork is a reality for about 25 percent of the North American working population. Night shift work has been associated to heart disease and cancer. a team of 44 scientists in 10 countries commissioned by the World Health Organization's International Agency on Cancer Research in 2007, had reported "limited" evidence of a connection between cancer and night shift work in people.
Sleep and the circadian system play an important role in cardiovascular health and antitumor activity. The International Agency for Research on Cancer (IARC) has concluded that "shiftwork that involves circadian disruption" is considered a Group 2A carcinogen and "probably carcinogenic to humans." The exact cause remains unknown but is believed disruption of the circadian system which is caused by exposure to light at night which can alter sleep-activity patterns, suppress melatonin production, and deregulated genes involved in tumor development.
In this new study a team of international researchers had examined possible links between rotating night shift work and all-cause, cardiovascular disease and cancer mortality of almost 75,000 registered U.S. nurses. The team used data from the from the Nurses' Health Study (NHS). The research team examined 22 years of follow-up.
The team had found that working rotating night shifts for over five years was linked to an increased risk of all-cause mortality and mortality from cardiovascular disease. Mortality from all causes appeared to be 11 percent higher for women with 6-14 or? 15 years of rotating night shift work. Cardiovascular disease mortality appeared to be 19 and 23 percent higher respectively. No association was found between rotating shift work and any cancer mortality with the exception of lung cancer with an increased risk of 25 percent among shift workers of more than 15 years.
Nurses' Health Study which is based at Brigham and Women's Hospital began in 1976 with 121,700 U.S. female nurses aged 30-55 years, who have been followed up with biennial questionnaires.
Night shift information was collected in 1988, at which time 85,197 nurses responded. Women with pre-existing cardiovascular disease or other than non-melanoma skin cancer were excluded. In total 74,862 women were included in this analysis. For this analysis rotating shift work was defined as working a minimum of three nights per month in addition to days or evenings in that month, the respondents had been asked how many years they had worked rotating night shifts. The pre-specified categories were never, 1-2, 3-5, 6-9, 10-14, 15-19, 20-29, and ?30 years.
Dr, Eve Schernhammer, MD, DrPH, Associate Professor of Medicine, Harvard Medical School
Associate Epidemiologist, Department of Medicine, Brigham and Women's Hospital, Assistant Professor of Epidemiology, Department of Epidemiology, Harvard School of Public Health commented about 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. 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."
She continues that comparing this work to previous studies 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."
This study appears in the current issue of the American Journal of Preventive Medicine