Breast Cancer Risk Increases With Disrupted Sleep Cycles

Tired night-shift worker. Diego Cervo/Shutterstock.

A new study into the sleep patterns of mice shows that there is a link between acute sleep disruption and an increased risk of developing breast cancer.

The study used mice with an altered gene that makes them more susceptible to breast cancer tumors. This can be likened to women who have underlying genetic factors that raise the risk of developing breast cancer.

The mice naturally gain mammary tumors after around 50 weeks. However, this period was reduced by eight weeks in mice that experienced disturbances in their circadian rhythm, the roughly 24-hour-long cycle in an organism's physiological processes. The disruption delayed the mice's body clocks by 12 hours every week.

There was also another not entirely unexpected side effect: The animals subjected to sleep cycle disturbances gained roughly 20% more weight than the mice with natural circadian sleep cycles.

"The conclusion is that chronic changes in light schedules are a driving factor for breast cancer development, weight gain, and other metabolic problems," said Bert van der Horst from the Erasmus University Medical Center and senior author of the study. The findings are published in Current Biology.

The investigation only involved mice, so it's hard to say whether the same applies to humans. However, the results of these findings are along the same lines as previous research into the relationship between cancer and disturbed sleep cycles in humans. 

Past studies have linked lifestyles that demand sleeping schedules that oppose the natural body clock with an increased risk of not just breast cancer but other types of cancer as well. These studies notice higher-than-usual incidences of breast cancer in flight attendants and night workers

There are speculated reasons for this link between defying the circadian rhythm and developing cancer. For example, melatonin production is suppressed when night workers are exposed to artificial light, the natural sleep cycle is disrupted, and essential vitamin D levels are reduced due to lack of sunlight. The researchers of the study accounted for melatonin and low vitamin D levels to show that they were not contributing factors to the development of breast cancer or weight gain in the mice. This further indicates that it is a disruption to the circadian rhythm that is responsible for the increased risk.

However, it is tricky to find a definite cause in humans since the lifestyles of day and night workers may vary in ways that affect the risk of cancer more than altered sleep cycles. This is why this research on mice is essential: It is difficult to do a long-term study in humans, whereas it is much easier to do the same in mice.

This research is prompting a call to the Netherlands (but also the rest of the world) to relax the number of consecutive night shifts that employees are required to work. It also warns women with an increased risk of developing breast cancer to avoid shift work altogether.

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