A large study in the United States has found nurses working rotating night shifts face an increased risk of cardiovascular disease and lung cancer.

The study of almost 75,000 registered nurses spanning 22 years shows nurses working rotating night shifts, of at least three nights a month along with day and evening shifts, for five or more years had a modest rise in all-cause and cardiovascular disease mortality.

The study, published in the American Journal of Preventive Medicine, also found nurses working rotating night shifts of 15 or more years had a modest increase in lung cancer mortality.

Researchers state the study is further evidence of the potentially detrimental effects of rotating night shift work on health and longevity.

An international team of researchers used data from the Nurses’ Health Study, which is based at Brigham and Women’s Hospital, and began collecting night shift data in 1988.

After excluding women with pre-existing cardiovascular or other than non-melanoma skin cancer, 74,862 women were included in the analysis.

Reviewing the 22 years of follow-up data, they found 14,181 deaths documented, with more than 3000 of those attributed to cardiovascular disease and more than 5400 to cancer.

Researchers discovered an 11 per cent rise in all-cause mortality for women with 6 to 14 years or more than 15 years of rotating shift work.

Cardiovascular-related mortality also appeared to increase 19 per cent and 23 per cent for these groups, respectively.

There was no association between rotating shift work and any cancer mortality, except for lung cancer in nurses who worked nights for 15 or more years - with a 25 per cent higher risk.

The World Health Organisation classified night shift work as a probable carcinogen in 2007 as a result of circadian disruption.

The study’s authors point to sleep and the circadian system for playing a vital role in cardiovascular health and anti-tumour activity.

“The circadian system and its prime marker, melatonin, are considered to have anti-tumour effects through multiple pathways, including antioxidant activity, anti-inflammatory effects, and immune enhancement,” it states.

“They also exhibit beneficial actions on cardiovascular health by enhancing endothelial function, maintaining metabolic homeostasis, and reducing inflammation.

“Direct nocturnal light exposure suppresses melatonin production and resets the timing of the circadian clock.

“In addition, sleep disruption may also accentuate the negative effects of night work on health.

“Taken together, substantial biological evidence supports the role of night shift work in the development of poor health conditions, including cancer, CVD, and ultimately, mortality.”

The study shows women with longer durations of rotating night shift work tended to be older, had a higher BMI, were more physically active after standardising for age and were more likely to be smokers, while they drank less alcohol and ate less daily cereal fibre compared to women without night shift work.

These nurses had also gained more weight since the age of 18 and were more likely to have a history of diabetes, hypertension and hypercholesterolemia.

Researchers state further studies are needed to explore “the potential for residual confounding or effect modification by smoking in relation to lung cancer mortality, and to confirm associations of night shift work with colorectal cancer, kidney cancer, and myeloma”.

“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,” the study states.