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Night shifts worsen diabetes risk for unhealthy women

Intermittent night shifts exacerbate effect of unhealthy behaviours on women’s type 2 diabetes risk

Louise Prime

Thursday, 22 November 2018

Women who have an unhealthy lifestyle have an even greater risk of developing type 2 diabetes if they also work intermittent night shifts, new research has shown. The authors of the large study* of female nurses, published today in the BMJ, said their findings suggest that most cases of type 2 diabetes could be prevented by adhering to a healthy lifestyle – and the benefits could be greater in rotating night shift workers.

The research team, led from Harvard T.H. Chan School of Public Health and Harvard Medical School in Boston, US, analysed data on 143,410 women participants in the Nurses’ Health Study (1988-2012) and Nurses’ Health Study II (1991-2013), who at baseline had been free from type 2 diabetes, cardiovascular disease, or cancer.

They compared rates of incident cases of type 2 diabetes between women with different lifestyle behaviours, and between those who did or didn’t work rotating night shifts (defined as at least three night shifts per month in addition to day and evening shifts in that month). ‘Unhealthy lifestyle’ included current smoking, physical activity levels <30 minutes per day at moderate to vigorous intensity, diet in the bottom three-fifths of the Alternate Healthy Eating Index score, and body mass index >25.

Over 22-24 years of follow-up, there were 10,915 incident cases of type 2 diabetes overall. The researchers calculated that the multivariable adjusted hazard ratios (HR) for type 2 diabetes were 1.31 per five-year increment of duration of rotating night shift work; 2.30 per unhealthy lifestyle factor (ever smoking, low diet quality, low physical activity, and overweight or obesity); and 2.83 for their joint effect, with a relative excess risk due to interaction of 0.20. The attributable proportions of the joint effect were: 17.1% for rotating night shift work alone; 71.2% for unhealthy lifestyle alone; and 11.3% for their additive interaction.

The study authors concluded: “Among female nurses, both rotating night shift work and unhealthy lifestyle were associated with a higher risk of type 2 diabetes. The excess risk of rotating night shift work combined with unhealthy lifestyle was higher than the addition of risk associated with each individual factor.

“These findings suggest that most cases of type 2 diabetes could be prevented by adhering to a healthy lifestyle, and the benefits could be greater in rotating night shift workers.”


*Shan Z, Li Y, Zong G, et al. Rotating night shift work and adherence to unhealthy lifestyle in predicting risk of type 2 diabetes: results from two large US cohorts of female nurses. BMJ 2018; 363: k4641. http://dx.doi.org/10.1136/bmj.k4641.

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