Many studies over the years have suggested that having an off-kilter circadian rhythm can cause a range of both physical and mental health problems. Going to bed at a decent time and waking up early always seems to be the healthiest sleep pattern. Being an early riser may even lower women’s risk of developing breast cancer, according to a recent study.
Dr. Rebecca Richmond, a research fellow in the MRC Integrative Epidemiology Unit at the University of Bristol in the United Kingdom and the Cancer Research U.K. Integrative Cancer Epidemiology Program, and a team of colleagues set out to learn more about the potential link between sleep cycles and breast cancer risk.
The team culled data from the U.K. Biobank project, a long-term study designed to answer questions regarding the genetic and environmental causes of breast cancer.
Richmond and her colleagues also accessed information that the International Breast Cancer Consortium (BCAC) had obtained from a genome-wide association study of breast cancer.
“Using genetic variants associated with people’s preference for morning or evening, sleep duration, and insomnia, […] we investigated whether these sleep traits have a causal contribution to the risk of developing breast cancer.”
The team presented their findings at the 2018 National Cancer Research Institute Cancer Conference in Glasgow, U.K.
Getting into the Research
In the study, more than 180,000 women self-reported whether they were “morning people” or “evening people.” 
The researchers also analyzed genetic variants linked to whether someone is a morning or a night person in more than 220,000 women. The team used a type of statistical model called Mendelian randomization to analyze the genes.
- Their calculations showed that people whose genes made them more likely to be morning people were as much as 48% less likely to develop breast cancer, compared to late risers.
- Similarly, using the self-reported data on 180,000 women, Richmond and her team found that early risers were about 40% less likely to develop breast cancer than their late-rising counterparts. Richmond chalked the variation up to technical differences.
- Women who self-reported sleeping more than 7-8 hours per night also had a slightly increased risk of breast cancer – an increase of 20% for every extra hour they slept, the Mendelian randomization analysis showed.
- Overall, 1 in 100 women who considered themselves morning people developed breast cancer, while 2 in 100 late risers developed the disease.
Important Things to Consider
However, the team noted that there are numerous factors involved in a person’s breast cancer risk, and these numbers don’t represent an absolute risk. Furthermore, the findings can’t be applied across populations, as the majority of women in the study were of European descent.
“Sleep is likely to be an important risk factor for breast cancer, but it isn’t as large as other well-established risk factors like BMI or alcohol.”
According to the American Cancer Society (ACS), 45% of U.S. cancer deaths are linked to modifiable risk factors, such as smoking, overweight and obesity, eating red and processed meat, and physical inactivity.
“We know that sleep is important generally for health. These findings have potential policy implications for influencing sleep habits of the general population in order to improve health and reduce risk of breast cancer among women.”
It’s also important to note that a woman’s sleep pattern may be indicative of underlying health problems or unhealthy lifestyle habits that may also influence breast cancer risk. For example, a 2017 study found that people who were fed a poor diet during childhood have a higher risk of breast cancer.
Experts say more research is needed, as the findings can’t be applied more widely.
Dipender Gill, a clinical research training fellow at Imperial College London, said:
“The statistical method used in this study, called Mendenial randomization, does not allow causality to be inferred. For example, the genetic determinants of sleep may also affect other neuronal mechanisms that affect breast cancer risk independently of sleep patterns. In such a scenario, sleep patterns may be associated with breast cancer risk, but not directly cause it.”
 CBS News Boston