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“Sleep is that golden chain that ties health and our bodies together” Thomas Dekker
Despite the fact that we spend about a third of our existence asleep, remarkably little is understood about its actual purpose. We are all under the control of a 24-hour sleep/wake cycle (or “circadian rhythm”). We have evolved over millions of years so that fluctuations in aspects of our physiology, such as digestion, immune function and hormone production, are synchronised to the day/night cycle.
In recent years, sleep has undergone a renaissance. We used to take pride in our ability to function on minimum rest (remember Margaret Thatcher and her four-hour sleep routine?). Nowadays, we better understand how sleep impacts on risk of disease, and more importantly how we can modify these risks. We have all experienced the restorative power of sleep, in terms of both physical and mental health. On the other hand, short sleep duration has been linked to increased risk of chronic illnesses including heart disease, stroke, and diabetes. But what about cancer? Conflicting reports have emerged over recent years on the subject of sleep and cancer.
Sleep and Cancer Risk
Disruption to our circadian rhythm affects cellular processes such as cell growth, DNA damage and gene expression. Sleep is a time when we most efficiently repair DNA damage, and strengthen our immune system. If this period of time when the body is busy fortifying its defences is chronically disturbed, there is reason to believe that this may have serious implications in terms of cancer risk.
Sleeping too little?
Short sleep duration also results in lower levels of melatonin, a hormone that promotes sleep but that also acts as a tumour suppressor. Ongoing sleep deprivation leads to chronic inflammation and insulin resistance.
The results from scientific research are mixed; some population studies show no significantly increased risk of cancer from insufficient sleep (<6 hours per night), but others identify elevated risks for several types of cancer, such as breast, bowel and prostate.
Sleeping too much?
There are also mixed results regarding “extended sleep” duration and cancer risk. This is typically defined as sleeping more than 9 hours per night.
This lack of consensus between studies reflects not just the intricacy of cancer as a disease, but also the complexity of sleep as a phenomenon. To study the relationship between the two would require close and rigorous long-term observation of sleep patterns and sleep quality; all difficult to measure accurately.
Sleep disorders and cancer
Some studies have identified links between sleep disorders and higher cancer risk. Most have involved patients with obstructive sleep apnoea, a disorder that leads to intermittent interruptions in breathing during sleep. As well as a greater risk of cancer, this condition was associated with more aggressive cancers and higher mortality rates from cancer in some studies. However, the results are again mixed, showing increased risk for melanoma, pancreatic and kidney cancers, but lower rates for breast, bowel and prostate cancers.
Night shift working “a probable human carcinogen”?
Shift workers work evenings, early mornings and throughout the night, awake at times when the body would naturally be sleeping. Instead they must sleep during the hours when the body (and society) is designed to be awake.
There is some evidence that those who work night shifts are at a slightly higher risk for developing several types of cancer. Most research has concentrated on breast cancer risk, mainly in specific occupations such as nurses, flight attendants and radio operators.
A study of women who worked long-term night shift schedules reported a 19% higher overall risk of cancer compared to those who didn’t participate in this work. Risks for some cancers such as breast and skin were even higher. The longer the duration of this working pattern, the higher the risk became. Following this, in 2007 the World Health Organisation identified night shift working as a “probable carcinogen”.
The theory is that exposure to light during the night adversely affects our internal “body clock”. This was tested in laboratory animals exposed to light at night, with the results suggesting a link with cancer. These findings were widely reported; in fact, in 2009 the Danish government offered compensation to night shift workers who later went on to develop breast cancer.
Taking all the evidence together, it is difficult to draw firm conclusions, as other risk factors affect the risk of cancer in night shift workers, such as obesity, smoking and alcohol intake. In addition, some studies differed in their definition of a night shift, and others did not include enough people to give definitive results. In 2016, data from observational studies conducted in 10 countries (including the UK) were pooled, and did not identify a link between shift work and breast cancer risk.
In 2018, a large study followed up 105,000 women exposed to varying amounts of domestic light at night. It concluded that there was no evidence that increased light exposure affected the risk of breast cancer.
Larks and Owls: Do “morning people” have a lower breast cancer risk?
A 2018 study from Bristol University of 400,000 women involved studying small differences in their DNA, they were able to categorise those who were genetically programmed to be early risers versus those programmed to be night owls. It reported that women who go to bed early and rise early (“larks”) were 40% less likely to develop breast cancer than those who stay up later and wake later in the morning (“owls”). However, although showing an association, it did not establish cause and effect, or suggest a plausible mechanism for the findings. Although the results are intriguing, there is not enough evidence to advise women to change their sleeping habits yet.
Our modern lifestyle and sleep
Lack of sufficient sleep has reached epidemic proportions in western societies. Hectic work and personal schedules, high stress levels, exposure to screens and artificial light at night are some of the hurdles we face.
What we can be certain of is that all of us, whether larks or owls, can stack the odds in our favour by exercising regularly, not smoking, drinking within sensible limits, and maintaining a healthy weight. As for sleep, striving to maintain a regular and good quality sleep schedule (in alignment with our natural body clock) seems sensible for now, but for many that is easier said than done.
Dr Samanatha Nimalasena is a Clinical Research Fellow in the Translational Breast Radiobiology team within the Division of Radiotherapy and Imaging
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