It’s a question I’m asked a lot: What is the connection between sleep and cancer?
People are often really asking: If I sleep poorly, does my cancer risk go up? That’s one important aspect of the relationship between sleep and cancer. But there are others, too. Sleep can be a key preventative strategy that may help reduce your risk of cancer. Thanks to scientific breakthroughs that have deepened our understanding of circadian rhythms, sleep is now being used as a therapeutic tool in the treatment of cancer. And for people living with cancer, sleep can be both a challenge and an opportunity—a challenge to sleep well, and an opportunity to use sleep to strengthen the body’s natural powers to push back against cancer.
In a series of articles, I’ll unpack and explore the complex relationship between cancer and sleep, and look at some of the cutting-edge scientific discoveries that are informing how we approach cancer prevention and making sleep an emerging player in the world of cancer therapy.
Sleep and cellular health
Before we jump into looking at how some specific sleep issues may influence cancer risk, let’s take a step back and examine some fundamentals about both sleep and cancer.
Sleep, we know, is absolutely essential to the body’s cellular health. During sleep—especially during deep, slow-wave, sleep—the body goes to work to repair damaged cells and DNA, promote healthy new cell growth, fortify and strengthen the immune system.
Research points to the deeply restorative power of sleep: A 2014 study found that getting enough high-quality sleep is associated with slower cellular aging in healthy adults. Scientists measured cellular age using telomere length, which is considered a key cell-age indicator. Older adults who slept enough and slept well had longer telomeres—a sign of “younger” cells.
Studies also highlight the risks to cell and DNA function in the absence of sufficient high-quality sleep. A 2014 study of rats found lack of sleep increases damage to DNA, injury and dysfunction to cells, including heightened cell death, increased cell proliferation, and increased risk for cell replication errors.
Why all this attention to sleep’s role in protecting cellular and DNA health? Cancer is a disease of many forms, but all types of cancer involve out-of-control growth and replication of damaged, abnormal cells. DNA plays an elemental role in cancer, because our genes exert control over how our cells behave, including how they grow, repair, and replicate.
We have a great deal to learn about how sleep affects the risks for and development of cancer. But it’s not difficult to imagine a fundamental connection between sleep—with its essential role as a time for the body to restore and maintain healthy cell function—and cancer.
With those fundamentals in mind, let’s take a closer look at how different sleep patterns and sleep issues may affect our cancer risk.
Does sleeping too little increase risks for cancer?
Lack of sufficient sleep is an epidemic problem in our society, one that’s been growing for decades. More than 1 in 3 American adults get less than the recommended minimum of 7 hours of nightly rest. Sleep deprivation and sleep debt are also rampant among teenagers, with as few as 15 percent of teens getting the 8-10 hours of nightly sleep they need. Short sleep has been scientifically linked to increased risks for serious and chronic illnesses including heart disease and stroke, obesity and type 2 diabetes. What about cancer?
The scientific research right now is mixed. A couple of recent reviews of studies have found no statistically significant increased risk of cancer from insufficient sleep. At the same time, other studies have shown a lack of sleep is tied to elevated risks for several different types of cancer. Studies have shown short-sleep duration—that’s another way of saying not sleeping enough—is linked to a higher risk of colorectal cancer. Insufficient sleep has also been identified as a factor in elevating risk for colorectal adenomas—these are polyps found in the colon, which can sometimes develop into cancer. And multiple studies, including this long-term, large-scale study published recently, indicate that short sleep may increase breast cancer risk, one of the most-studied forms of cancer.
Can sleeping too much increase cancer risk?
The scientific picture is similarly mixed when it comes to oversleeping and cancer risk. The health risks of oversleeping tend to be overlooked, compared to not sleeping enough. Oversleeping is associated with depression and other mood disorders, obesity, cardiovascular disease, and neurodegenerative diseases such as Alzheimer’s. As with short sleep, there’s not yet a clear picture of how extended sleep duration—typically defined as sleeping more than 9 hours a night—impacts cancer risk.
Some of the reviews of studies that show no elevated risk from short sleep also show no elevated cancer risk connected to long sleep. And yet there are studies indicating links between specific types of cancer and sleeping too much. Research in postmenopausal women showed an increased risk of liver cancer associated with sleeping more than 9 hours a night. Some research shows breast cancer risk may rise with longer sleep duration, while other studies investigating breast cancer risk found no link to long sleep duration. In particular, the risks of estrogen-positive forms of breast cancer appear to be increased by sleep duration.
Why this lack of clarity about the impact of sleep amounts on cancer risk? Sleep is a tremendously complex phenomenon. Cancer is a tremendously complex disease. Tracking and attributing the effects of one on the development of the other is a difficult endeavor, one that requires close, rigorous, long-term observation. We need to continue to search for better, more thorough answers about how sleep duration influences the development of cancer.
For many aspects of health, it’s not just sleep duration—or quantity—that matters, but also sleep quality and the timing and routine of sleep. And there’s compelling research suggesting that disrupted, poor quality, and irregularly-timed sleep may have a significant impact on cancer risk.