How Sleep and Breast Cancer Are Related

A recent study in the UK looked at the correlation between sleep schedules (chronotypes) and the risk of breast cancer.

By Sheryl Grassie

The BMJ, a prestigious peer-reviewed medical journal previously called the British Medical Journal, published a study in June of 2019 that looked at sleep variables and level of risk for developing breast cancer.

The study assessed three variables: insomnia, sleep duration, and chronotypes (sleep schedules whether early or late). The most significant findings from the research suggests that early risers, or self-termed “morning people,” have a lower risk of developing breast cancer. A second finding suggested that sleeping more than 7-8 hours a night was also statistically associated with increased risk, but less so. This research data is good news if you are a morning person, but there is more to the story, so read on.

Young woman wake up in the morning and sitting on bed at window door side relaxing in holiday with sunlight, back view

Study Parameters

Research studies don’t happen in isolation, but often are designed to replicate or expand on previous studies. This study did just that by expanding on a prior research project by the same group of academics. The prior study showed that early risers had a lower risk of cardiovascular disease mortality. Conversely, they found an increased risk of illness for night owls. Expanding on the notion that early risers have a lower risk for health issues they targeted breast cancer.

The study, called “Larks, owls, and breast cancer” for the connection to the terms early risers (larks) or late night people (night owls), was a large scale study with a sample size of 400,000 women. The participants were drawn from two databases, a UK Biobank and the Breast Cancer Association Consortium. They were assessed for the previously mentioned variables, insomnia, sleep duration, and chronotypes.

The research focused heavily on underlying genetic factors and used a research methodology called Mendelian randomisation. This process looks at the relationship between non-static risk factors and disease by assessing genetic differences. This means that the underlying genetics that cause us to sleep longer, have insomnia, or be an early riser or night owl may be the real link to the health issue. Researchers have found genes that are a factor in determining our circadian rhythms, and whether we are an early riser or not.

Findings From The Study

There were a number of findings that come from this study, and a number of previously held assumptions that were reinforced. Breast cancer is the most common type of cancer in the UK, and is prevalent in the US. The American Cancer Society reports that an estimated one quarter of a million invasive cases will be diagnosed in 2019. The connection between breast cancer and sleep is multi-layered and multi-factorial, but the following specifics were generated by the study.

  • Statistical differences: 1 in 100 early risers had breast cancer; 2 in 100 night owls were diagnosed.
  • No link to insomnia: The one variable in the study that did not draw a correlation was insomnia. Having insomnia is not a risk factor for breast cancer.
  • Increased risk with more sleep: Those participants who slept longer than the recommended 7- 8 hours nightly has an increased risk of getting breast cancer. This may be related to other factors like increased weight. More sleep can equal more weight and obesity is a risk factor in breast cancer.
  • Don’t change your sleep habits: Although early risers have a lower risk for developing breast cancer, it may be genetically determined, both the sleep schedule and the risk, so changing sleep habits won’t help. Further, other factors like weight and alcohol consumption are still considered the primary risk factors for breast cancer.
  • Sleep and breast cancer are linked: Sleep affects weight and health. Excess weight and poor health can contribute to cancer. More research is needed to determine exactly how sleep more fully correlates to breast cancer.


How is sleep and breast cancer related? Women who wake early have a lower risk of developing breast cancer. It is believed that sleep schedules, either early or late, are determined by genetics, which may also play into the risk of getting cancer. Researchers do not recommend altering your sleep schedule to get up early as a way to reduce cancer risk. They do, however, continue to validate the connection between sleep and health.

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