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The accidental discovery of a pattern in blood samples taken from cancer patients has determined that metastatic breast cancer spreads most aggressively at night.

Hormones that control circadian rhythms appear to affect the rate of metastasis. “When the affected person is asleep, the tumor awakens,”  says lead researcher Nicola Aceto, professor of Molecular Oncology at the Swiss Federal Institute of Technology in Zürich. Cells that leave the tumor at night were also found to divide more quickly and therefore have a higher potential to form new tumors.

Credit: Bloom, by Bianca Capaldo and Caleb Dawson Breast Cancer Laboratory, Walter and Eliza Hall Institute of Medical Research

“Our research shows that the escape of circulating cancer cells from the original tumor is controlled by hormones such as melatonin,” says Zoi Diamantopoulou, the study’s first author.

The findings have the potential to change the way doctors take samples and biopsies, including a new emphasis on time of day, and to suggest different therapeutic approaches in the future.