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.The new research provides insights into biological mechanisms that allow cancer to spread, and an invaluable observation that physicians can use to track cancer growth and metastasis by taking blood samples on precise schedules.“We clearly demonstrate that the timing of a biopsy is very critical for better diagnosis,” says Zoi Diamantopoulou, a cancer cell biologist in Aceto’s lab and the study’s lead author.Scientists made a point of clarifying that the study does not show that cancers are caused by sleeping or rest. Whatitshows is that once a cancer has established, its progression is affected by the sleep and related hormonal changes. “For patients who already have cancer, consistent sleep is vital to ensuring that the rest of their body is strong enough to endure treatment and fight off the disease,” says Harrison Ball, a graduate student at University of Michigan, who was invited to write a commentary on Aceto’s study.Sleep promotes a strong immune system which is good for protecting against cancers. “Our immune system is at its best when we get enough rest,” says Sunitha Nagrath, a chemical engineer at University of Michigan Ann Arbor, who develops tools for isolating and studying rare cells from cancer patients. “Cancer might spread aggressively [during the night], but the body can fight back if we have a great immune system.”The study’s conclusions fill a critical gap in knowledge in cancer biology, “especially finding that CTCs tend to be more numerous and more aggressive at night,” says Francis Lévi, a medical oncologist at France’s Paris-Saclay University, who has spent the last three decades exploring how circadian rhythms influence health and disease. Lévi’s own contributions include showing that the toxicity and side effects of certain cancer drugs could be reduced by altering the time they are administered—a field now known as chronotherapy—and establishing that these times differed for men and women.“This will still take decades and probably prospective clinical trials to make clinicians realize that time of day has an importance,” says Christoph Scheiermann, an immunologist at University of Geneva in Switzerland. But based on this study, sampling blood at the right times can be of immediate diagnostic importance, Scheiermann believes.The sentence“Cancer might spread aggressively [during the night], but the body can fight back if we have a great immune system.”Can be best restated to …