Breast cancer spreads at night, new research shows, which could help target the spread of the disease. Tumours are most active while women sleep - making daytime therapy less effective.
Researchers found they are fuelled by the hormone melatonin - produced in response to darkness. The findings also have implications for when breast biopsies are carried out.
Project leader Professor Nicola Aceto explained: "When the affected person is asleep, the tumour awakens."
A study of 30 patients showed more cancerous cells were generated at night. It also applied to mice. The disease was most aggressive during the day - as they are nocturnal animals.
First author Dr Zoi Diamantopoulou said: "Our research shows the escape of circulating cancer cells from the original tumour is controlled by hormones such as melatonin - which determine our rhythms of day and night."
Melatonin is produced by the pineal gland - a pea-sized organ at the base of the brain. It dictates our sleep patterns. The phenomenon may influence examinations of patients by oncologists.
Results could be affected depending on the time in which tumour or blood samples are taken. It was a serendipitous discovery along these lines that put the Zurich-based scientists on the right track.
Prof Aceto said: "Some of my colleagues work early in the morning or late in the evening - sometimes they'll also analyse blood at unusual hours."
The Swiss Federal Institute of Technology team noticed samples taken at different times of the day had very different levels of circulating cancer cells. Another clue was the surprisingly high number of cancer cells found per unit of blood in mice compared to humans.
The reason was the lab rodents sleep during the day - which is when researchers collect most samples. To date, little attention has been paid to the question of when tumours spread - or metastasise. It was previously assumed they release such cells continuously. But the study in Nature turns the idea on its head - showing they mainly arise during the sleep phase of affected individuals.
Prof Aceto said: "In our view, these findings may indicate the need for healthcare professionals to systematically record the time at which they perform biopsies. It may help to make the data truly comparable."
Around 56,000 women are diagnosed with breast cancer in the UK each year - making it the most common form. It claims 11,500 lives annually. Globally, there are around 2.3 million new cases - and almost 700,000 deaths - every year. The time medications should be taken to improve their effectiveness is an increasing area of interest. It could revolutionise patient care. Body clock-related changes have been identified in the actions of more than 100 drugs.
