Your body clock and breast cancer risk

From sunrise to sunset and all hours in-between, our body clocks are attuned to the rhythms of the day. Now, more research is showing that the same system that controls our circadian rhythms can also protect us from developing cancer.

According to a 2018 study, a protein known as human period 2 protein usually works in tandem with daily circadian rhythms, and interacts with tumour suppressor proteins in cells to control cell division. But when the body clock is disrupted, such as working the night shift, this protein is unable to prevent cells from dividing at certain times of the day. Researchers found that while healthy cells work on the same circadian rhythm as the rest of the body, tumours often have a different rhythm – they divide differently than healthy cells, and at different times. Disrupted sleep patterns (including shift work, short sleep duration or exposure to light at night) have been linked to the development of breast cancer, though there is limited evidence.

Another 2014 study saw researchers keep a group of rats with breast cancer in a light environment for 12 hours, followed by 12 hours of complete darkness. Another group spent 12 hours in light and 12 hours in very dim light – similar to the light peeking in from underneath a door in an otherwise dark room. They discovered that the tumours of rats who spent time in dim light grew 2.5 times faster than rats who were kept in the dark. Plus, when researchers attempted to shrink these tumours with tamoxifen, the tumours in rats who had some light at night continued to grow at the same rate. Rats in the control group, in comparison, saw their tumours markedly shrink.

However, when researchers gave the rats a nightly supplement of melatonin – a hormone that helps our body know when to sleep and wake up – both groups showed similar tumour growth and responsiveness to treatment. The findings suggest that exposure to light at night – which can alter the production of melatonin – could be responsible for increased cancer growth.

Meanwhile, a study from 2016 found that melatonin slowed the spread of HER-2 positive breast cancer cells, by stopping a signal that causes healthy cells to develop into tumour cells.

Research is currently focusing on whether circadian rhythms may influence how effective cancer treatment is. Using human cancer cells – many of which have been donated by women with breast cancer – researchers are testing whether radiation therapy is more effective when given at certain times of the day. Researchers hope that in the future those with cancer may be able to receive treatment at times that make it more effective for treating and targeting cancer cells.