While cancer research has paid little attention to the question of when tumors shed metastatic cells, the researchers discovered that circulating cancer cells that form metastases arise during sleep.

The study, which included 30 female cancer patients and mouse models, found that the tumor generates more circulating cells when the organism is asleep. In addition, the study showed that cells that leave the tumor at night also divide more quickly and therefore have a higher potential to form metastases, compared to circulating cells that leave the tumor during the day.

"Our research shows that the escape of circulating cancer cells from the original tumor is controlled by hormones such as melatonin, which determine our rhythms of day and night," said Zoi Diamantopoulou, the study's lead author and a postdoctoral researcher at ETH Zurich.