Short Sleep, The Immune System and Cancer
Have you ever wondered what would happen if you stopped sleeping entirely? Perhaps the best way to illustrate the importance of sleep is to detail what happens in its absence. In 1983, scientists at the University of Chicago devised an ethically provocative experiment to find out what near total sleep deprivation would do to rats. The rats were paired and placed on flat, circular disks above a pool of water. The pair were separated by a barrier. One of the rats was constantly monitored by EEG. If the rat being monitored fell asleep the disk would begin slowly spinning, meaning that both rats would wake up in order to prevent themselves from falling into the water. This meant that the rat being monitored had a 90% reduction in sleep, while the control rat who was free to sleep while the other rat was awake, experienced only a 30% reduction in sleep.
They found that the rat suffering a 90% reduction in sleep would die in 15 days on average. That’s quicker than from total food deprivation (starvation). The cause of death was unclear, although there were outward signs of a declining immune system. Sores had begun to cover the rat’s skin and tails, suggesting that their body could no longer fight off even the most basic infections. A full post mortem examination of the rat’s organs confirmed that the damage inflicted on the body by sleep deprivation was far from limited to the exterior. The immune system had become so weak that bacteria which naturally inhabits the gut had spread into the bloodstream leading to a systemic bacterial invasion which ravaged the body from the inside out, eventually killing the rats.
Most of us rarely miss an entire night of sleep, let alone 15 days. But studies suggest that, collectively, we are sleeping less than we once did. After all, life has a way of getting in the way, doesn’t it? Losing a few hours here binge-watching your favourite show, an early morning for a flight there. Could this more subtle erosion of our sleep be impacting our immune system too? This is a question that Dr Aric Prather at the University of California, San Francisco took upon himself to answer. He recorded the sleep of 164 healthy men and women for 7 days. Then participants were exposed to the common cold; a dose of rhinovirus was squirted directly up their nose’s. Prather then monitored the participants to see how many of them became objectively infected to see whether there was any correlation between time spent asleep and susceptibility to the common cold. He found a linear relationship between sleep and rate of infection, with short sleepers at a significantly elevated risk of catching the common cold. Those who slept less than 5 hours per night had an almost 50% probability of catching a cold, while those who slept 7 hours or more had just an 18% chance.
These results are consistent with a study involving nearly 57,000 nurses over 4 years, examining the relationship between short sleep and pneumonia. They found that the individuals sleeping 8 hours per night had the lowest risk of pneumonia, those sleeping less than 5 hours a night had a 70% increase in the risk of pneumonia. Considering the impact conditions like the common cold and pneumonia have on the health care system, these findings are best not ignored. In fact, a study conducted by the University of Michigan in 2003 found that in the U.S alone, the common cold had an annual cost of 40 billion dollars. However, the immune suppression effects caused by poor sleep and circadian disruption have been linked to far more serious effects, including decreasing the efficacy of vaccines and increasing the probability of a growing list of cancers.
Sleep can decrease the efficacy of vaccines for the people who need them most. In 2002 a ground breaking study demonstrated that the flu vaccine was less effective for individuals who don’t sleep sufficiently. In the experiment, one group had their sleep restricted to 4 hours per night for six nights, while the other group was allowed to sleep a full 7.5-8 hours per night. After the 6 days, all participants received a flu vaccine. In the days following, the researchers took blood samples to measure how effective the individuals were at generating an antibody response. Those individuals who were sleep-deprived had an immune response that was half that of those who were sleeping sufficiently. Similar results have since been demonstrated with the Hepatitis A and B vaccines.
Humans are made up of roughly 50 trillion cells, which are constantly dividing and multiplying. Sometimes during these divisions, the DNA is damaged and cells mutate into cancerous cells. Thankfully your body has evolved a complex system to kill these cells and prevent them from multiplying further. Accounting for 10-15% of your white blood cells, natural killer cells are unique in that they can kill your own cells if they become cancerous or infected with a virus. Studies suggest that disruption to your circadian rhythm can compromise your body’s natural defence to cancer. For example, one landmark study from the University of California found that just a single night of sleep restricted to 4 hours resulted in a 70% reduction in the number of circulating natural killer cells, compared with a full eight hours sleep. If an acute reduction in sleep can have such a devastating impact, it is easy to imagine how this could increase one's risk of cancer when sleep loss is extended over months or even years.
A study of nearly 24,000 women found that individuals who slept 6 hours or less per night were 1.6 times more likely to develop breast cancer than those sleeping 7 hours or greater. Since then, further studies have found a correlation between short sleep and increased risk for prostate and colon cancer also. Another study published in the BioMed Central Cancer Journal studying how circadian disruption impacted the growth of tumours. They divided rats into two distinct groups; one group of rats was exposed to light during the day and darkness at night, recreating a natural day/night cycle. The second group of rats was given constant light exposure for five weeks, disrupting their circadian rhythms. They then inoculated the rats with tumour cells and tracked the tumour growth. They found that the tumours in the rats exposed to constant light grew faster and were denser than those with a light/dark cycle, suggesting that circadian disruption creates a favourable environment for tumour growth.
In Denmark, the Government began paying compensation to women who developed breast cancer after long periods doing shift work, following the review of the epidemiological evidence. While more research is needed to definitively say sleep deprivation alone can cause cancer, the link was strong enough for The World Health Organisation to recognise circadian disruption as a probable carcinogen. And as I always say, if it’s good enough for the W.H.O. then it’s good enough for me.
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