In addition, the researchers found that the CTCs shed during rest divided faster than those shed during waking hours. Rest-phase CTCs expressed the genes for mitotic cell division more than other CTCs, making them better able to metastasize or spread. Cells in the primary breast tumors also multiplied faster during rest periods.

Dr. Kotryna Temcinaite, senior research communications manager at U.K. charity Breast Cancer Now, told Medical News Today:

“These interesting findings suggest that tumor cells are more likely to spread at night and these cells are also better able to successfully establish themselves in other parts of the body than the cancer cells that leave the tumor during the day.”

The researchers suggest that hormones, such as melatonin, testosterone and insulin, may be involved in this night-time spread and division, but stress that more research is needed to discover how.

The finding that CTCs are both more numerous and divide faster at night may have implications for both the diagnosis and treatment of breast cancers.

Dr. Nicola Aceto, professor of molecular oncology at ETH Zurich, who led the study, stressed the importance of timing to ensure data is comparable:

“In our view, these findings may indicate the need for healthcare professionals to systematically record the time at which they perform biopsies,” he said.

And Dr. Baker agreed: “Though this research is at an early stage, it indicates that breast tumors shed more cancer cells at night. Scientists need to do more studies to confirm this, and find ways to make sure we get the most informative liquid biopsy results by keeping the time of day in mind.”

Her comments were echoed by Dr. Temcinaite: “While this discovery adds to the growing body of evidence about whether the time of day treatment is given to patients is important, the researchers only looked at patterns of breast cancer spreading and what influences this process in mouse models and analyzed cancer cells in the blood in a small number of patients at different times.”

“We look forward to further research to understand more how timing can influence diagnosis or treatment decisions,” she added.

Prof. Aceto and his team plan to investigate whether other cancers behave in a similar way, and whether altering the timing of existing therapies may make them more effective.