A study presented this week at an international cancer conference indicates that unhealthy sleep patterns may contribute to the development and progression of breast cancer.
Chinese researchers at Jilin University Second Hospital (in northeast China) analyzed data from more than 159,000 women who participated in the UK Biobank, a large database collecting health and lifestyle information since 2005. Among the 7,291 women who developed breast cancer over a 15-year period, the researchers determined that poor sleeping habits were associated with a 10% increase in the risk of developing breast cancer. the disease, according to findings presented in Hong Kong during the World Congress of the International Union for Cancer Control.
The results do not demonstrate a direct cause of breast cancer, but suggest various biological changes related to poor sleep patterns, including alterations in immune and inflammatory responses, may be an important factor. The study authors were not available for comment.
Dr. Harold Burstein, a medical oncologist at Dana-Farber Cancer Institute, estimated that sleep disturbances may result in one additional case of breast cancer for every 200 women. Burstein, who was not involved in the study, called the findings “provocative.”
The new research adds to growing evidence about potential health risks associated with nighttime light exposure and sleep disturbances.
For Kevin Nead, associate professor of breast epidemiology and radiation oncology at the University of Texas MD Anderson Cancer Center, the study is another confirmation of the importance of maintaining basic but fundamental lifestyle habits.
“I love seeing studies like this,” says Nead, who was also not involved in the research. “There is strong evidence to show that disrupted sleep, abnormal sleep patterns, and short sleep are definitely not good for your health.”
How sleep may be related to cancer risk
White blood cells, the body’s main cells responsible for fighting cancer, function following a 24-hour circadian rhythm. This means that the quality of our immune defenses is synchronized with the light and dark cycle.
Therefore, experts suspect that people who habitually go to bed late, when the body normally expects to be asleep, and who are exposed for a longer time to artificial light at night, They have immune systems less capable of detecting and eliminating cancer cells.
“Basically, the problem is the light,” explained Dr. Larry Norton, a breast cancer oncologist at Memorial Sloan Kettering Cancer Center, who was not involved in the study. “Exposure to light at times of day when you should not be exposed to light can have detrimental effects.”
According to Norton, another plausible explanation could be related to melatonin, the hormone primarily known for promoting sleep, which also plays a lesser-known role: being a tumor suppressor.
Previous research has shown that people who sleep fewer hours have lower levels of melatonin.
“Melatonin is an important hormone that helps us maintain a normal circadian rhythm,” explains Norton. “It is regulated by the hypothalamus, an area of the brain extremely sensitive to light cycles. Therefore, the alteration of melatonin is one of the theories that try to explain this relationship.”
(Sleeping with light may raise your risk of heart disease, study says)
The problem may not be related to total sleep duration, that is, how many hours a person sleeps per night.
An analysis based on the Million Women Study, a large investigation of women’s health in the United Kingdom, found no relationship between sleeping less than six hours or more than nine hours a night and the risk of breast cancer.
On the other hand, irregular sleep patterns and constant alterations in circadian rhythms and hormones such as melatonin may constitute a more important warning signal.
Separately, a 2025 study based on the renowned Nurses’ Health Study showed that nurses who regularly alternated between day and night shifts had twice the risk of developing breast cancer than those who never worked nights.
Shift workers, whose routines tend to be less stable, frequently face sleep disturbances.
“There is very strong evidence that those who work at night, especially those who alternate between night and day shifts, appear to have a higher incidence of breast cancer,” Norton said. “Interestingly, people who work constantly at night for many years do not seem to experience this effect with the same intensity. Therefore, the constant interruption of biological rhythms is what is more important than simply working day or night.”
According to Norton, this lack of consistency can prevent the body from adapting properly. It can also affect the so-called clock genes, regulated by light exposure and responsible for coordinating the activation and deactivation of other genes that control fundamental biological functions.
“The alteration of clock genes may be related to the changes we see when cells become cancerous,” he said.
Healthy Sleep Habits May Offer Protection
The main risk factors for breast cancer include family history, inherited genetic mutations, having started puberty before age 12 or menopause after age 55, having dense breast tissue, and being over 50 years old. However, women have some control over other risk factors, such as a sedentary lifestyle, being overweight, and excessive alcohol consumption.
Even so, women who work at night or who experience frequent sleep interruptions should not be alarmed.
Postmenopausal women, who are the group at highest risk for breast cancer, are also the most likely to experience sleep problems, so the observed relationship does not necessarily imply direct causality, Burstein explained.
Also Irregular sleeping habits may be part of an overall less healthy lifestyle, which in turn could increase cancer risk.
“Disturbed sleep leads to poor eating habits, which in turn promote weight gain, and excess weight is associated with breast cancer,” Nead said. “There are many ways in which lifestyle factors interact with each other, making these relationships complex.”
A 2019 British study, also based on data from the UK Biobank, found that people with a morning chronotype, that is, those who wake up and go to bed early, have a lower risk of developing the disease.
Nead said simple measures, such as going to bed early and at the same time each night, reducing nighttime distractions and avoiding caffeine at the end of the day, can make a big difference.
“I think the number of people who actually do all those things is very low,” he concluded.
