Women who are naturally early riders turned out to be at lower risk of developing breast cancer than evening types in a new study. One in 100 women who considered herself in the morning developed breast cancer compared with two out of 100 women who called themselves at night people.
Cancer risks associated with a person's body bell and sleep patterns have been reported in earlier research and the British researchers wanted to investigate sleeping properties in detail, as well as all genetic factors that underlie this.
Self-reported preferences for mornings or evenings (according to their own definition of that preference) were recorded in more than 180,000 women, led by Dr. Rebecca Richmond, researcher at the Cancer Research UK Integrative Cancer Epidemiology Program and Epidemiology Unit at the University of Bristol, and presented Tuesday at the NCRI Cancer Conference in Glasgow.
Richmond's team also analyzed genetic variants linked to whether someone is a morning or night person in more than 220,000 women to find out if these can help cause causation of breast cancer.
This type of statistical model, called Mendelian randomization, showed that people whose genes made them more likely to be early stigters were less likely to develop breast cancer by as much as 48%, as evidenced by the 220,000 participants in the study.
The second analysis, using self-reported sleep data from 180,000 participants, showed a similar trend of early-rising women who had a 40% lower risk of breast cancer. The variation is due to technical differences, Richmond noted.
Women who reported themselves to sleep more than the average seven to eight hours per night also found a slightly increased risk of breast cancer, with 20% per extra hour sleeping, according to the team's Mendel randomization analysis.
But the team points out that many factors are involved in a person who develops breast cancer and that these numbers are not an absolute risk. The finding can not be applied to populations as the majority of women included each of European ancestors.
"Sleep is probably an important risk factor for breast cancer, but it's not as big as other well-established risk factors like BMI or alcohol," says Richmond.
The American Cancer Society says that 45% of cancer deaths in the United States are linked to modifiable risk factors, such as cigarette smoke, excess body weight, eating red and processed meat and physical inactivity. About 4% of American cancer deaths were linked to drinking alcohol and breast cancer now charity warns that some alcohol intake increases the risk of breast cancer. And obesity will be the leading preventable cause of breast cancer in women in Britain, according to a report from earlier this year.
"We know sleep is important in general for health," Richmond said. "These results have potential political implications for affecting sleep habits in the general population to improve health and reduce the risk of breast cancer among women."
Our circadian rhythms, or body clocks, control bodily functions such as sleep patterns, blood pressure and metabolism and in disturbances can increase the risk of cancer and other diseases.
Richmond's team conducted its genetic analysis hoping to dig in the possible causes and consequences of this link. However, experts warn that more research is needed and that existing results can not be applied to a greater extent.
"The statistical method used in this study, called Mendelian randomization, does not always allow the causal link to be read," said Dipender Gill, clinical research education colleague at Imperial College London. "For example, the genetic determinants of sleep may also affect other neuronal mechanisms that affect the risk of breast cancer regardless of sleep patterns. In such a scenario, sleep patterns may be associated with the risk of breast cancer, but not directly cause it."
Stephen Burgess, a doctoral student at the University of Cambridge, added that a mechanism for explaining the link is also not known or understood.
"The authors do not show any biological mechanism, with which sleep-time preferences can affect the risk of breast cancer. Another limitation is that the chronotype is self-reported, and the survey did not specifically recruit individuals with different sleep patterns, such as night shift workers," Burgess wrote in the comments in the study.
The study has not yet been published in a medical journal. Gill added that it has not yet been reviewed by other experts in the field.
Read: Sleeping: Do you get enough?
Not as big as other factors
Richmond stressed that the 48% lower risk was identified in "extreme" cases where people identified themselves as "determined" morning people from the five categories that they could choose from – determined morning, more morning than evening, no more evening than morning, decided evening
"Sleep is probably an important risk factor for breast cancer," she said. But it is not as big a risk factor as others well established, such as weight or alcohol consumption, added Richmond.
The nightwalls should not be worried about the results, said Richmond. "I would not support women to get up earlier to reduce the risk of breast cancer."
There are theories about the causes of sleep's effects on cancer, she says, for example, the idea that artificial light at night leads to hormone interruption.
Dr Sowmiya Moorthie, senior police analyst in epidemiology at the PHG Foundation, who was not involved in research, added that the study's great strength is the use of "multiple methods for investigating the relationship between sleep pathways and breast cancer, which allows researchers to show consistency in their results . "
"As far as the impact of research is concerned, existing evidence supports sleep patterns affecting cancer risk, but it is still unclear how individual early or late uptake preferences interact with actual sleep behaviors," wrote Moorthie in an email.