Well-Being Medical Advances

Breast cancer spreads the most at a particular time of day, scientists find

"When the affected person is asleep, the tumor awakens," a leader of a new study said.
Andrii Kalenskyi/ iStock

Story at a glance


  • Researchers discovered cancer cells leaving a tumor at night divide more quickly and are more likely to spread than cells leaving the tumor site during the day. 

  •  They said study findings could change the way breast cancer is diagnosed and treated.

  • Breast cancer is the second most common form among women in the U.S.

Breast cancer spreads most efficiently at night as the cells that later spread mainly appear while a person is asleep, a new study suggests.  

“When the affected person is asleep, the tumor awakens,” the study’s leader Nicola Aceto, a professor of molecular oncology at ETH Zurich, said in a statement.  

Researchers discovered cancer cells leaving a tumor at night divide more quickly and are more likely to spread than cells leaving the tumor site 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 postdoctoral researcher at ETH Zurich.  

The researchers, who took blood samples from participants at various times throughout the day, further noted that study findings may change the way breast cancer is diagnosed and treated. 

“In our view, these findings may indicate the need for healthcare professionals to systematically record the time at which they perform biopsies,” Aceto said. “It may help to make the data truly comparable.” 

The study, published in the journal Nature, included 30 female cancer patients and mouse models. 


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Breast cancer is the second most common form among women in the U.S. The latest data from the Centers for Disease Control and Prevention shows there were 264,121 diagnosed cases among women in 2019 alone. Around 42,000 women and 500 men die annually in the U.S. from the disease.  

Health professionals advise maintaining a healthy weight, avoiding or limiting alcohol and exercising regularly all while communicating with one’s physician about family history of breast cancer and when to start screenings.  

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