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(NaturalNews) A study on almost 24,000 Japanese women recently published in the British Journal of Cancer has found that lack of sleep can greatly increase the risk of breast cancer, with women who slept 6 hours or less every night having a significantly higher risk.

Breast Cancer Statistics

Breast cancer is the most common cancer to hit women worldwide. In Japan, when age-standardized to the world population, the incidence rate was 28.3 per 100,000 in 1991, and rose to 39.5 in 2001.

In the United States in 2004, the disease hit more than 185,000 women and over 1,800 men, with almost 41,000 women and 362 men dying from it that year. In that year, after non-melanoma skin cancer, breast cancer was the next highest cancer killer of American women. It was also their fifth highest killer overall.

Next up, over to Canada, where, among the women, breast cancer is the most common type of cancer to strike. According to Canadian Cancer Society estimates, about 22,400 women will be diagnosed with the disease this year, with about 5,300 dying from it.

With such grim statistics, every little thing which can be done to prevent and combat the disease becomes all the more critical.

Details of Study

The Ohsaki National Health Insurance (NHI) Cohort Study started in 1994 and involved 28,515 women in northeastern Japan. The questionnaire used included information on sleep duration and other lifestyle habits.

Participants who had withdrawn from the NHI study before follow-up, had a history of cancer, did not provide information on their sleep duration, and who reported having slept for less than 4 hours or more than 12 hours every night were omitted. This left the data for 23,995 women to be analyzed. An 8-year period, from 1995 to 2003, was used, during which 143 women were hit with breast cancer.

Findings of Study

The women who slept 7 hours each night was used as the reference group. It was then found that women who slept 6 hours or less each night had a 62% higher risk of getting breast cancer. On the other hand, those who slept 9 hours or more every night had a 28% lower risk of getting the disease.

It would follow, then, that those who slept 6 hours or less every night had 2.25 times the risk of getting breast cancer when compared to those who slept 9 hours or more each night.

The results remained largely consistent even when participants who were diagnosed with breast cancer within 3 years from the start of the study were excluded, or when the data was analyzed by age and menopausal status.

Previous Studies

The findings of this study validates the findings of two previous prospective cohort studies relating breast cancer and sleep duration (Verkasalo et al, 2005, Wu et al, 2008). Those two studies had also shown a significant decrease in breast cancer risk for those who slept the longest.

It must be noted, though, that another such study (Pinheiro et al, 2006) did not find any such association. The study team pointed out, however, that that study had looked at residential nurses, who underwent rotating-shift work and had varying sleep timings. The findings of that study thus might not be applicable to the general population.

Strengths and Limitations of Study

According to the study team, their research had a couple of strong points. Firstly, it used study subjects from the general population, thus allowing for overall generalization of its findings. In addition, it used the Miyagi Prefectural Cancer Registry, which the study team said is "one of the earliest and most accurate population-based cancer registries in Japan".

There were also, however, several limitations. Firstly, self-reported sleep data was used, and assessment was also only carried out once. In addition, and probably very significantly, no information on sleep quality, sleep timing, use of sleep medication, or presence of sleep disorders were available. These factors, of course, are very important as they can directly or indirectly affect cancer risk.

Further, the researchers added that they had no information with regard to rotating-shift work or night work, but they felt that would not have affected their findings greatly as more than half of the study subjects were housewives, farmers or retired.

The Sleep Duration – Breast Cancer Link

Why is breast cancer risk linked to sleep duration? The answer could lie in melatonin, which is secreted during night sleep. When a person sleeps fewer hours, less melatonin is secreted, and lower levels of the chemical had previously been associated with increased breast cancer risk.

In addition, melatonin may possess an inhibitory effect on gonadal function, which includes synthetizing and secreting sex hormones. It had also been found to have an antiproliferative effect on breast cancer cells.

The Bottom Line

If the findings from this study are indeed accurate, then there is an immense difference in breast cancer risk between sleeping 4 to 6 hours every night, and just sleeping 1 to 3 hours more each night. In fact, it is more than likely that the protective effects of sufficient sleep also extend to other forms of cancer. 7 hours of sleep a night may thus be a good number to aim for.

Hopefully, in time to come, further research will reveal more information relating sleep and disease risk, with sleep quality and sleep timing being two of the main possibilities.

Main Source

Sleep duration and the risk of breast cancer: the Ohsaki Cohort Study (http://www.nature.com/bjc/journal/v...)

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