Insomnia can double the risk of prostate cancer in men, a study has shown.
The risk rises proportionately with the severity of sleep problems, researchers found, increasing from 1.6 to 2.1 times the usual level.
Why poor sleep can affect men's chances of developing the disease is unexplained. But a previous link has been seen between insomnia and breast cancer in women.
Insomnia ups the risk of prostate cancer among men
"Sleep problems are very common in modern society and can have adverse health consequences," said study leader Dr Lara Sigurdardottir, from the University of Iceland in Reykjavik.
"Women with sleep disruption have consistently been reported to be at an increased risk for breast cancer, but less is known about the potential role of sleep problems in prostate cancer."
Each year around 40,000 men in the UK are diagnosed with prostate cancer and 10,000 die from the disease.
The researchers studied more than 2,000 men aged 67 to 96 who were questioned about their sleeping habits. Specifically, they were asked if they took sleeping pills, had trouble falling asleep, found it difficult to get back to sleep after waking in the night, or woke early and stayed awake.
Among the participants, between 8.7% and 5.7% reported severe and very severe sleep problems. None had prostate cancer at the start of the study, which continued for five years.
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During this time, 6.4% of the men were diagnosed with prostate cancer. Compared with men who had no problems sleeping, those suffering from insomnia were significantly more likely to develop prostate cancer.
The association was stronger for advanced stage disease. For men with "very severe" sleep problems, the risk of being diagnosed with advanced prostate cancer was more than tripled.
The researchers took steps to rule out the possibility that sleep problems were caused by undiagnosed prostate cancer or enlarged prostate glands, which can cause an urge to urinate during the night.
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The findings are published in the American Association for Cancer Research journal Cancer Epidemiology, Biomarkers & Prevention.
Dr Sigurdardottir added: "Prostate cancer is one of the leading public health concerns for men, and sleep problems are quite common. If our results are confirmed with further studies, sleep may become a potential target for intervention to reduce the risk of prostate cancer."