MedWire News: Women who regularly sleep for 6 hours or less a night are more likely to develop breast cancer than those who get more sleep, study results indicate.

Dr Masako Kakizaki, from Tohoku University Graduate School of Medicine in Sendai, Japan, and team studied data on around 24,000 women, aged 40–79 years, who completed a health and lifestyle questionnaire, which included questions on sleep duration.

The participants were then divided into four groups based on their average sleep duration – 6 hours or less, 7 hours, 8 hours or 9 or more hours of sleep each night.

During the 8-year monitoring period, 143 of the women were diagnosed with breast cancer.

The researchers found that, compared with women who slept for an average of 7 hours a night, those who slept for 6 hours or less a night had a 62% increased risk of developing breast cancer, and those who slept for 8 hours had a 14% increased risk of the disease.

Interestingly, women who slept for 9 hours or longer were 28% less likely to develop breast cancer than those who slept for an average of 7 hours a night.

Writing in the British Journal of Cancer, Dr Kakizaki and colleagues conclude: “We have found a significant inverse association between sleep duration and breast cancer risk in Japanese women, those who slept 6 hours or less having a significantly increased risk.”

They suggest that, among women who get little sleep, reduced levels of the sleep hormone melatonin may result in increased production of female hormones, which are needed for the development of some types of breast cancer.