Study shows breast cancer spreads more at night

A team of researchers from ETH Zurich, Switzerland, found that breast cancer cells spread to other parts of the body most effectively during the night period, while the patient sleeps. The study, published in the journal Nature on Wednesday (22/6), suggests that the treatments would work best when applied later in the day.

The research investigated 30 women with breast cancer, nine of whom had metastatic cancer and were not undergoing any treatment. Blood samples were taken at 4 am and 10 am. The results showed that 78% of circulating tumor cells (CTC) were found in the dawn sample.

The scientists did similar tests on mice that were transplanted with different types of breast cancer, and found that, depending on the tumor, between 87% and 99% of CTCs were found overnight. These cells were also clumping together (which increases the chances of forming a new tumor) about 278 times more in the dawn sample.

“Most cancer treatments are not designed to attack the tumor cell at a specific time. They are thought to take into account that the tumor is there and should be hit any minute. Now, we understand better and we know that the treatment can be improved”, explains scientist Nicola Aceto, one of the authors of the study, in an interview with the website New Scientist.

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He states that the results make sense: while the immune system respects the circadian cycle, decreasing its action while the individual sleeps, the same does not happen with cancer, which reproduces at another rhythm.

Scientists say the finding raises several other questions, including whether there is a specific time in sleep when tumor cells develop most easily, or whether patients should sleep more or less to optimize treatment.

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