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HomelifeDoc Talk: The link between sleeping too little, or too much, and...

Doc Talk: The link between sleeping too little, or too much, and cancer

SINGAPORE – Too much sleep increases your risk of dying from cancer.

I bet that statement caught your attention. That a relationship exists between sleep and cancer is not a concept far from the minds of cancer patients, and the public in general.

However, our intuition tends to lead us to conclude, not necessarily backed by science, that it is the lack of, rather than excessive, sleep that is associated with increased cancer risk.

Well, it is time to wake up to a new scientific finding: Sleeping too much is a problem too, at least where cancer risk is concerned.

In April 2022, Japanese scientists published their findings in the medical journal Cancer Epidemiology: Sleeping more than 10 hours a day (versus seven hours) increases the risk of dying from cancer by 18 per cent in men and 44 per cent in women.

This is a fairly sizeable study covering 5.9 million person-years (number of participants in the study multiplied by the number of years these participants have been followed up).

And in case you think that the finding in this study is an oddity in this area of medical research, it is not. This result is consistent with an earlier study in 2013 conducted in the United States among doctors. It concluded that sleeping more than nine hours a day (versus seven hours) is associated with a 35 per cent increase in colorectal cancer risk in men, particularly pronounced in those who are overweight or who had a snoring problem. An increase was seen in women too, though to a lesser degree.

Having said that, many of my patients who blamed their lack of adequate sleep in the past being a contributing factor to them developing cancer are also standing on firm scientific ground.

A European study in 2012 showed that sleeping less than six hours a day (versus seven to eight hours) is associated with a 43 per cent increase in risk of cancer.

If sleeping too much or too little is bad for cancer prevention, how are Singaporeans doing in their “sleep score”?

Worldwide statistics on sleeping patterns released in 2021, timed to coincide with World Sleep Day, showed that Singaporeans are leaning towards being sleep-deprived.

While the world averaged 7.15 hours of sleep a day, Singaporeans managed to clock only 6.6 hours on weekdays, with a little bit of catching up during the weekends and a final average score of 6.8 hours daily.

If we accept the conclusions from the clinical studies above, what could be the scientific rationale for too much or too little sleep leading to cancer?

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The mode of action likely relates to the disruption of normal circadian rhythm and/or poor quality of sleep.

Circadian rhythm describes the normal day-night and sleep-wake cycles of the body. A normal circadian rhythm is important in maintaining normal hormonal production and function of the immune system.

Abnormal hormone production has been implicated in pathways that drive, or fail to suppress, cancer cells formation. Impairment of normal immune function, on the other hand, has been implicated in how the body fails to eliminate potentially cancerous cells during their formative stages.

Disruption of the circadian rhythm may potentially affect people’s metabolism, making them more susceptible to carcinogens. Indeed, animal models have demonstrated greater cell damage and DNA replication errors in sleep-deprived laboratory animals. Cell damage and errors in the DNA codes of cells lead to genetic mutations, the basic mechanism that drives cancer cell transformation.

Sleeping excessively, on the other hand, has been associated with greater secretion, by the body, of inflammatory cytokines. Chronic inflammation is a possible pathway in the promotion of cancer growth.

Research focusing on the impact of disruption of normal sleep-wake circadian rhythm from a common phenomenon in any modern society, that of working night shifts, adds credence to the theory that messing with the normal circadian rhythm can have dire consequences.

Studies have shown that nurses and airline cabin crews who do shift work on a regular basis have an elevated breast cancer risk. Exposure to artificial light at night is thought to suppress the secretion of a hormone with a potential cancer-suppression effect, known as melatonin, by the pineal gland of the brain.

The International Agency for Research on Cancer, an organisation under the umbrella of the World Health Organisation, rates working night shifts as a probable carcinogen.

If exposure to artificial light at night is potentially harmful, it begs the question of whether the habit of some folks of sleeping with the lights on at night is unwittingly increasing their risk of cancer?

The research finding on this question is a mixed bag. A Spanish study in 2018 found that sleeping with the lights on at night is associated with an increase in the risk of prostate cancer, but paradoxically, associated with a decreased risk of breast cancer. The jury is still out.

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What of the quality of sleep?

Experiments in laboratory mice have demonstrated that fragmented sleep promotes accelerated tumour growth. While medical ethics preclude the possibility of similar studies on human subjects, studies on patients with a common medical condition, that of obstructive sleep apnoea, offer an insightful look into the possible impact of poor sleep quality on humans with respect to cancer risk.

Obstructive sleep apnoea is a condition where the relaxation of the muscles of the upper airway during sleep leads to intermittent collapse of the passage, causing pauses in breathing and hence fragmented sleep. An increase in cancer rate is often observed in such patients.

While the exact mechanism of cancer formation is still debatable, it is conceivable that apart from fragmented sleep, the lack of oxygen (known as hypoxia) during the pauses in breathing creates an environment conducive to cancer growth.

Individuals with obstructive sleep apnoea may sometimes sleep for long hours, but the poor quality of that sleep, unfortunately, is not made up for by the longer sleep time.

In other words, a loud snorer may have more problems than just an irritable spouse on his hands.

Apart from the risk of developing cancer, clinical data exists to support a relationship between sleep pattern and the prognosis of patients already afflicted with cancer.

An American study in 2017 involving nurses with breast cancer showed that sleeping more than nine hours a day was associated with greater cancer mortality. Another study done earlier in 2014 concluded that sleep misaligned with circadian rhythm was associated with earlier cancer relapse in breast cancer patients.

While a number of studies on sleep-wake circadian rhythms and cancer have revealed several danger zones, several others have shown how such an understanding may be exploited to improve cancer treatment efficacy.

Immunotherapy, a mode of treatment that mobilises the body’s own immune system to fight cancer, has started to show discrepancy in efficacy depending on the time of day that the treatment takes place.

Studies looking at the treatment outcomes of lung cancer and melanoma (cancerous skin mole) patients on immunotherapy, demonstrated that survival is generally higher in patients who regularly receive their treatment in the early morning when compared with similarly treated patients who mostly received theirs in the late afternoon. It appears that syncing immunotherapy with the patients’ circadian rhythm leads to a more robust immune response.

In fact, it has been postulated that similarly synching the timing of vaccination with the circadian rhythm, giving the shots in the morning, may boost vaccine efficacy.

The relationship between sleep and cancer is not just a simple question of more sleep versus less.

For the average Joe (and Jane) keen to tap the cancer preventive potential of a healthy lifestyle, there is such a thing as too much or too little when it comes to sleep. The sweet spot seems to be between six and nine hours a day.

Are you hitting the sweet spot? That is something for you to sleep on.

Dr Wong Seng Weng is the medical director and consultant medical oncologist at The Cancer Centre.

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