Picture of a cluster of cancerous cells, which appear almost like a cluster of fuzzy and dimpled grapes.

Breast cancer spreads more aggressively during sleep

Many more cancerous cells circulate in the blood at night, suggesting that time of day could play a role in diagnosis and treatment.

This electron microscope scan shows a circulating tumor cell (CTC) cluster seen within the filters of a microfluidic device used to process blood samples. Every cell of the cluster comes from the same patient. This metastatic cell cluster, which is smaller than a full-stop in a magazine, floats through the blood and settles in healthy tissues, allowing the cancer to spread.  
Photograph by Martin Oeggerli

More than just a 24-hour biological timer that lulls us asleep at night and awakens in the morning, the inbuilt master clock in the human brain also controls the daily ebb and flow of hormones, regulates body temperature, cues hunger, and schedules digestion, among hundreds of physiological functions. Now a new study of breast cancer patients reveals that cancer cells take advantage of these hormone cycles to spread while the patient is sleeping.

Cancers spread to new locations when cells break away from the original tumor and travel to faraway tissues through blood or lymph systems after escaping across blood vessel walls. This spread or “metastasis” causes most deaths from cancers. Scientists used to think that these free circulating tumor cells, or CTCs, are shed continuously throughout the day into the bloodstream. But a new study shows that in breast cancer patients most CTCs are released during late phase of sleep just before sunrise rather than during the active day hours.

“When the patient is asleep, the tumor awakens,” says Nicola Aceto, a molecular oncologist at the Swiss Federal Institute of Technology (ETH) in Zurich, Switzerland, who led the study.

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