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updated 5/7/2013 6:29:08 AM ET 2013-05-07T10:29:08

Men who have a hard time falling asleep and staying asleep may be at an increased risk of prostate cancer, according to a new study from Icelandic and American researchers.

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The new study published Tuesday in the journal Cancer Epidemiology, Biomarkers and Prevention showed an association between disturbed sleep and not only an increased incidence of prostate cancer but also the severity of prostate cancer.

An association does not mean that disturbed sleep causes the cancer, only that the two appear linked, though the paper’s authors do speculate that one mechanism may involve the brain chemical melatonin. Some research shows melatonin production at night can help suppress cellular processes involved in cancer growth. Light and disturbed sleep inhibits melatonin production.

Scientists began exploring a possible link between sleep and cancer in the 1990s. In 2001, researchers from Harvard Medical School reported that among women participating in the Nurse’s Health Study -- one of the largest ongoing surveys of health in the world -- those who had worked rotating night shifts appeared “to have a moderately increased risk of breast cancer.”

By 2007, The International Agency for Research On Cancer designated disruption of normal light/dark and sleep cycles, as often occurs with shift work, a probable carcinogen, not unlike certain chemicals.

That designation remains somewhat controversial, and subsequent studies have sometimes contradicted each other. This new study will add to that debate.

The team, led by Lara Sigurdardottir at the Centre of Public Health Sciences, University of Iceland, surveyed 2,102 men for sleep patterns. They weren’t really looking for men who simply stayed up late and rose early, but those who experienced sleep disturbances like difficulty falling asleep, staying asleep, and falling back asleep if they woke up during the night.

Of the 2,102 men, 755 who did not have such disturbances formed the control group. In all, during a weighted average of five years follow-up, 135 men developed prostate cancer, 26 of them an aggressive form of the disease.

When the cases were plugged into a formula of “person-years” (the time each participant contributed to the study) “we found that men with sleep disruption were at increased risk of prostate cancer, particularly advanced prostate cancer, when compared with men who did not report any sleep problems,” the team reported.

They calculated a two-fold increased risk. In other words, Sigurdardottir explained to NBCNews.com in an interview, “let’s imagine that in a given time period of five years, each participant contributed the same amount of time to the study and the same number of participants were in each group. If 6 men out of 100 developed prostate cancer in the group without sleep problems, we would expect 12 out of 100,” or six more cases, in the sleep problems group.

According to the Centers for Disease Control and Prevention, out of 100 men age 60, about 15 will be diagnosed with prostate cancer over the next 30 years, so this is a significant, but not enormous, increase in risk.

Still, while cautioning against making too strong of a cause-effect link from the results, Sigurdardottir argued that “sleep disruption is a plausible cause factor.”

Matthew Cooperberg, an assistant professor of urology, epidemiology and biostatistics at the University of California San Francisco, told NBCNews.com that the study was interesting and possibly important, especially since it study found significantly more aggressive cancers in the disturbed sleep group.

But, he said, “we cannot say insomnia causes prostate cancer. Whatever is causing the insomnia may be causing the cancer.”

For example, he said, depression and stress may create disturbed sleep. But they also can lower the immune response. Animals that are chronically stressed get more cancers.

The study did try to control for a variety of known health risks, like smoking, body mass index, and recent medical diagnoses but it’s impossible to eliminate all possible confounding factors, so the researchers could not control for, say, whether a man experienced extra stress and for how long.

Many people try to take melatonin supplements as sleep aids, but both Sigurdardottir and Cooperberg said they don’t recommend it. Cooperberg said men would be better off seeing a doctor to try to get at a root cause of their disturbed sleep and Sigurdardottir explained that “supplements do not replace the amount that the” brain produces during the night.

Drugs like Valium and Ambien aren’t necessarily a long-term solution either, they said. They can have varying effects on melatonin production for one thing, and it’s unclear whether such drugs could play any role in disarming whatever trigger might be in play with the sleep-cancer link.

The only sure advice, Cooperberg, said, is that everybody ought to try to get a night’s rest.

Brian Alexander (www.BrianRAlexander.com) is co-author, with Larry Young Ph.D., of "The Chemistry Between Us: Love, Sex and the Science of Attraction," (www.TheChemistryBetweenUs.com), now on sale.

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