A recent Japanese study, published by the British Journal of Cancer in September this year, has found that women who get six or less hours sleep a night are much more likely to develop breast cancer than the rest of the population.
The study involved 23,995 Japanese women aged between 40 and 79 who were followed for an eight year period. Over the course of the study, 143 women developed breast cancer. The researchers found that women who slept for 6 or less hours a night were 62% more likely to develop breast cancer compared to women who slept for an average of 7 hours a night. 8 hours of sleep a night was associated with a slightly increased risk of breast cancer when compared to 7 hours however this result was not statistically insignificant. 9 or more hours of sleep a night was associated with a 28% reduction in breast cancer risk when compared to 7 hours sleep.
It is believed that melatonin, which is produced by the pineal gland to regulate the circadian rhythm, may reduce the risk of breast cancer by suppressing the release of the hormone estrogen. Estrogen stimulates the growth of ER-positive breast cancers which have estrogen receptors on the surface of the cancerous cells. Almost two-thirds of all breast cancers are ER-positive.
Melatonin production is increased under dark conditions and during sleep. Night shift workers, women who sleep in a relatively light environment (> 1 lux), and women who sleep for less than six hours during the night tend to produce less melatonin and are at an increased risk of developing ER-positive breast cancer. A study, published in the journal Epidemiology in 2001, found that the odds of developing breast cancer were 50% greater in women who worked at night time for at least half the year.
Women who have been diagnosed with ER-positive breast cancer should ensure they get a minimum of 7 hours sleep a night in dark conditions because melatonin can considerably slow the growth of ER-positive breast tumors. Synthetic melatonin supplements are often used by women suffering from breast cancer however scientific evidence on their effectiveness has been mixed to date.
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