Men who work a rotating shift pattern may be at increased risk of prostate cancer, research suggests.
Shift work might disrupt hormone secretion
Japanese scientists found that staff working rotating shifts were four times as likely to develop the disease as those working day or night shifts.
But British experts said the findings were far from conclusive.
The University of Occupational and Environmental Health study, of more than 14,000 workers, features in the American Journal of Epidemiology.
Shift work has also been linked to an increased risk of breast and bowel cancer.
The study also found that night shift workers were at a slightly increased risk of prostate cancer, compared with those who only worked days.
The researchers suggest the key may be reduced secretion of the hormone melatonin, which the body uses to induce sleep.
Melatonin has also been shown to have anti-cancer effects.
Reduced secretion of melatonin has been linked to increased production of sex hormones, which play a role in regulating prostate tissues.
In normal circumstances, secretion of the hormone is low during daytime, increases soon after the onset of darkness, peaks in the middle of the night, and gradually falls until morning.
Henry Scowcroft, of the charity Cancer Research UK, said: "There have been several previous reports that disturbances to natural body rhythms might be linked to cancer, and this report adds to that evidence.
"But it has never been shown that the actual sleep disturbance itself is responsible for the slight increase in risk observed in these studies.
"It might be that people with abnormal sleep patterns are more likely to be doing something else, such as smoking or eating unhealthily, that increases their risk."
Mr Scowcroft said the researchers only found 31 cases of prostate cancer over 10 years.
"There is a long way to go before we can say for sure whether sleep disturbance is linked to prostate cancer."
Chris Hiley, of the Prostate Cancer Charity, said the research was interesting - but not necessarily relevant to British men.
"This research certainly doesn't indicate that rotating shift patterns mean it's more likely that you will die from prostate cancer - the research paper doesn't talk about an increased risk of dying at all.
"Neither does it say that shift patterns cause prostate cancer.
"The researchers do speculate about the possible role of melatonin in the development of prostate cancer, but there's nothing here that allows us to be confident that a new important risk factor has been identified.
"The researchers themselves suggest more research is needed to see if this finding occurs in other places to the same or a greater degree - and only then, if it does, might this translate into sound advice to men."