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Breast Cancer Spreads More Aggressively at Night, Finds New Study

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A new finding about breast cancer has left scientists perplexed. Cancer cells may spread more efficiently at night, when the body is at rest and the person is sleeping. Researchers call this a “marked acceleration” in the way breast cancer metastasizes, or spreads in the body. This pattern of spread holds important implications for both the breast cancer diagnosis and treatment of breast cancer.

“When the affected person is asleep, the tumor awakens,” said lead author Nicola Aceto, professor of molecular oncology at ETH Zurich.

Published in the journal Nature last week, the research offers “startling” clues about identifying and further treating breast cancer, which remains plagued by a lack of timely and appropriate diagnosis. “If doctors detect breast cancer early enough, patients usually respond well to treatment. However, things become much more difficult if cancer has already metastasized. Metastasis occurs when circulating cancer cells break away from the original tumor, travel through the body via blood vessels, and form new tumors in other organs,” the researchers at ETH Zurich explained. Notably, breast cancer accounts for 14% of cancers in Indian women, and experts have observed a higher caseload in recent years which makes monitoring breast cancer a key piece of cancer treatment practices.

It begs the question: when does the tumor shed metastatic cells in the body?

The answer was borne by observing an irregularity. Researchers scrutinizing breast cancer cells in mice, at first, observed a peculiar trend: the tumor cells in circulation differed when analyzed at different times of the day. The said mice with a higher number of cancer cells were also those sleeping during the day, when the samples were collected. The researchers from ETH Zurich then looked at breast cancer in 30 women, a small data set, including 21 patients with early breast cancer and nine with stage IV metastatic cancer. The researchers went on to replicate this experiment four more times on different mouse models of breast cancer, again sampling the blood while the mice rested during the day and were active at night.

The blood samples presented a “striking and unexpected pattern.” A higher number of tumor cells (78.3%) was found in samples taken at night, as compared to blood samples drawn during the day.

The cancer cells collected during the rest period were “highly prone to metastasize, whereas circulating tumor cells generated during the active phase are devoid of metastatic ability”, the study noted. Evidently, then, cancer cells that were more in circulation during sleep cycles were also more aggressive.

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