Men who work at night may be at increased risk for prostate cancer (PCa) and other malignancies, according to a Canadian study.

Marie-Élise Parent, PhD, of INRS-Institut Armand-Frappier, a component of the University of Quebec in Laval, and colleagues analyzed data from a population-based case-control study conducted in Montreal between 1979 and 1985 that elicited information about job histories, including work hours, from 3,137 men with incident cancer at one of 11 anatomic sites and from 512 controls.

Compared with men who never worked at night, those who had ever worked at night had a 2.8 times increased PCa risk after adjusting for potential confounders, Dr. Parent and her colleagues reported online ahead of print in the American Journal of Epidemiology. They had a twofold increased risk for colon, rectal, and pancreatic cancers and non-Hodgkin’s lymphoma, as well as a 76% and 74% increased risk of lung and bladder cancer, respectively. The researchers observed no elevated risk with increasing duration of night work.

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The investigators noted that night work might influence cancer risk possibly via suppression of melatonin release as a result of exposure to light at night. Such suppression, Dr. Parent’s group pointed out, has been associated with disruption of circadian rhythms as well as a reduction in non-specific anti-cancer effects of the pineal gland (which produces melatonin), and an increase in reproductive hormone levels.

Previous research has found that disruption of melatonin secretion increases both tumor growth and the occurrence of mammary carcinomas in rodents. The International Agency for Research on Cancer recently designated shift work as probably carcinogenic in humans, they noted.

Unlike some other studies, Dr. Parent and her colleagues ascertained night work based on a complete lifetime history of night work as reported explicitly by each subject, so they could compute the lifetime cumulative duration of night work exposure for each man. Unfortunately, they noted, the questionnaire used in the study did not collect data on light intensity levels, type of shifts or other aspects of night work, and individual characteristics (e.g., morning person vs. night person).

In an accompanying editorial, Parveen Bhatti, PhD, and collaborators at the Fred Hutchinson Cancer Research Center in Seattle, said the results of the new study “provide further support for an etiologic role of night-shift work in cancer.”

“The results were robust to a number of sensitivity analyses, providing some reassurance that the results are not seriously biased in some manner,” they wrote.