Damage done by just one night’s bad sleep can leave you vulnerable

Why sleep really is the best medicine: Neuroscientist reveals how the damage done by just one night’s bad sleep can ‘dwarf’ what that of a lack of exercise and leave you vulnerable to heart disease and strokes 

Matthew Walker is one of the world’s leading experts on sleep. He was born in Liverpool and graduated in neuroscience and neurophysiology in Britain before going on to become a professor at Harvard University, where he became fascinated by the science of why we sleep. He moved on to the University of California, Berkeley, where he is currently professor of neuroscience and psychology, and director of the Centre for Human Sleep Science.      

Do you think you got enough sleep this past week? Can you recall the last time you woke up without an alarm clock feeling refreshed and not in desperate need of caffeine?

If the answer to either of these questions is ‘No’, you are not alone. Two-thirds of adults throughout all developed nations fail to get the recommended eight hours of sleep a night.

Do you think you got enough sleep this past week? Can you recall the last time you woke up without an alarm clock feeling refreshed and not in desperate need of caffeine?

I doubt you are surprised by this fact — but you may be surprised by the consequences.

Routinely sleeping less than six or seven hours a night demolishes your immune system and more than doubles your risk of cancer.

Insufficient sleep is a key lifestyle factor determining whether or not you will develop Alzheimer’s disease. Even moderate reductions in sleep for just one week disrupts blood sugar levels so profoundly that you would be classified as pre-diabetic.


Routinely sleeping less than six or seven hours a night demolishes your immune system and more than doubles your risk of cancer

Short sleeping increases the likelihood of your coronary arteries becoming blocked and brittle, setting you on a path towards cardiovascular disease, stroke and congestive heart failure.

Sleep disruption further contributes to all major psychiatric conditions, including depression and anxiety. It can even lead to suicide.

Perhaps you have also noticed a desire to eat more when you’re tired? This is no coincidence. Too little sleep swells concentrations of a hormone that makes you feel hungry, while suppressing a companion hormone that signals food satisfaction.

Despite being full, you still want to eat more. It’s a proven recipe for weight gain in sleep-deficient adults and children alike.

The danger of ‘sleep debt’ 

Setting aside the extreme cases of sleep deprivation, how do you know whether you’re routinely getting enough sleep?

While a clinical sleep assessment is needed to thoroughly discover the answer, an easy rule of thumb is to ask yourself these two simple questions.

First, after waking up in the morning, could you fall back to sleep at 10am or 11am?

If the answer is yes, you are probably not getting the sufficient quantity and/or quality of sleep.

Second, can you function optimally without caffeine before noon?

If the answer is no, then you are likely to be self-medicating your state of chronic sleep deprivation. You should take both of these signs seriously. They are usually due to not giving yourself adequate time for sleep — at least eight or nine hours in bed.

When you don’t get enough sleep, you then carry that outstanding sleepiness throughout the following day.

Like a loan in arrears, this sleep debt will continue to accumulate. You cannot hide from it.

The debt will roll over into the next payment cycle — and the next, and the next — causing a prolonged, chronic and potentially dangerous sleep deprivation.

This results in a feeling of chronic fatigue, manifesting in many forms of mental and physical ailments that are now rife throughout industrialised nations.

Other questions that can reveal signs of insufficient sleep are: if you didn’t set an alarm clock, would you sleep past that time? (If so, you need more sleep than you are getting.)

And: do you find yourself at your computer screen reading and then re-reading (and perhaps re-reading again) the same sentence? (This is often a sign of a fatigued brain.)

When you add up all of the above health consequences, a proven link becomes easier to accept — the shorter your sleep, the shorter your life span. The old maxim ‘I’ll sleep when I’m dead’ is therefore rather unfortunate.

And it is no coincidence that countries where sleep time has declined most dramatically over the past century, such as the U.S., the UK, Japan, South Korea, and several other nations in western Europe, are also those suffering the greatest increase in rates of the aforementioned physical diseases and mental disorders.

Scientists such as myself have even started lobbying doctors to start ‘prescribing’ sleep. As medical advice goes, it’s perhaps the most painless and enjoyable to follow.

There does not seem to be one major organ within the body, or process within the brain, that isn’t enhanced by sleep (and impaired when we do not get enough).

That we receive such a bounty of health benefits each night should not be surprising.

After all, we are awake for two-thirds of our lives, and we don’t just achieve one useful thing during that time, but rather myriad accomplishments that promote our well-being and survival.

Why, then, would we expect sleep — and the 25 to 30 years, on average, it takes from our lives — to offer one function only?

Sleep dispenses a multitude of health-ensuring benefits — and they’re yours to pick up as a repeat prescription every 24 hours, should you choose.

It enriches a diversity of functions within the brain, including our ability to learn, memorise and make logical decisions.

Sleep also recalibrates our emotional brain circuits, allowing us to navigate the social and psychological challenges of the next day with cool-headed composure.

We are even beginning to understand the most impervious of all conscious experiences: the dream.

Dreaming provides a unique set of benefits, including soothing painful memories and inspiring creativity, as I will be explaining next week.

Downstairs in the body, sleep restocks the armoury of our immune system, helping to fight malignancy, preventing infection and warding off all manner of illnesses.

Sleep also reforms the body’s metabolic state by fine-tuning the balance of insulin and circulating glucose. It regulates our appetite, helping to control body weight by encouraging healthy food selection rather than impulsive choices.

It is also needed to maintain a flourishing microbiome within your gut, where so much of our nutritional health begins.


Insufficient sleep is a key lifestyle factor determining whether or not you will develop Alzheimer’s disease. Even moderate reductions in sleep for just one week disrupts blood sugar levels so profoundly that you would be classified as pre-diabetic

Adequate sleep is also closely tied to the fitness of our cardiovascular system, lowering blood pressure while keeping our hearts in fine condition. (See the box overleaf for more information.)

It is difficult to imagine any other state that offers a more powerful redressing of physical and mental health. I love sleep (and not just my own, though I do give myself a non-negotiable eight-hour opportunity for it each night).

I want to discover all that remains unknown about it, and I hope to find any and all methods for reuniting humanity with the sleep it so desperately needs.

I was once fond of saying: ‘Sleep is the third pillar of good health, alongside diet and exercise.’ I have since changed my tune. Sleep is more than a pillar — it is the foundation on which the other two health bastions sit.

A balanced diet and exercise are vital, yes. But we now see sleep as the pre-eminent force in this health trinity.

Take the bedtime health test… 

Even if you are giving yourself enough time to get a full night of shut-eye, next-day fatigue and sleepiness may still occur if you are suffering from an undiagnosed sleep disorder.

There are now more than 100 recognised sleep disorders, with the most common being insomnia.

This is followed by sleep-disordered breathing, or sleep apnoea. Signs of this can include heavy snoring.

Should you suspect your sleep, or that of anyone else, is disordered, resulting in daytime fatigue, impairment or distress, speak to a doctor immediately and seek a referral to a sleep specialist.

Here’s a simple questionnaire from Dr Daniel Buysse, from the University of Pittsburgh, called the SATED questionnaire.

If you have a low score, then you’re more likely to suffer from a sleep disorder.

How to score: Rarely/Never (0); Sometimes (1); Usually/Always (2) 

 1. Satisfaction: Are you satisfied with your sleep?

2. Alertness: Do you stay awake all day without dozing?

3. Timing: Are you asleep (or trying to sleep) between 2am and 4am?

4. Efficiency: Do you spend less than 30 minutes awake at night? (This includes the time it takes to fall asleep and awakenings from sleep.)

5. Duration: Do you sleep between six and eight hours per day?

your total…………………

0 = Poor sleep health

10 = Good sleep health

The physical and mental impairments caused by one night of bad sleep dwarf those caused by an equivalent absence of food or exercise. Take away the bedrock of sleep, or weaken it just a little, and careful eating or physical exercise become less than effective.

Recall the last time you had the flu. Miserable, wasn’t it? Runny nose, aching bones, sore throat, heavy cough and a total lack of energy. You probably just wanted to curl up in bed and sleep. As well you should.

Your body is trying to sleep itself well. Sleep fights against infection and sickness by deploying weaponry from your immune arsenal and cladding you with protection.

When you do fall ill, the immune system actively stimulates the sleep system, demanding more bed rest to help reinforce the war effort. Reduce sleep for even a single night, and that invisible suit of immune armour is rudely stripped from your body.

To investigate this, my colleague Dr Aric Prather at the University of California, San Francisco, measured the sleep of 164 healthy men and women for a week. He then quarantined them, and proceeded to squirt a good dose (or rather a live culture) of rhinovirus, which causes the common cold, straight up their noses.

Dr Prather then kept them in the laboratory for the following week, monitoring them intensely.

There was a clear, linear relationship with infection rate and sleep: the less sleep an individual was getting in the week before facing the active common cold virus, the more likely it was that they would catch a cold.

In those sleeping five hours on average, the infection rate was almost 50 per cent. In those sleeping seven hours or more a night, the infection rate was just 18 per cent.

Infectious illnesses, such as influenza and pneumonia, are among the leading causes of death in developed countries. Doctors and governments would therefore do well to stress the importance of getting sufficient sleep during the flu season.

Perhaps you are the sort of responsible person who gets a flu jab each year, boosting your own resilience while adding strength to the immunity of the herd — your community. However, that flu jab is only effective if your body actually reacts to it by generating antibodies. A remarkable discovery in 2002 demonstrated that sleep profoundly impacts your response to a standard flu vaccine.

In the study, healthy young adults were separated into two groups. One had their sleep restricted to four hours a night for six nights, and the other group got between seven-and-a-half and eight-and-a-half hours in bed each night.

Why heart attacks spike when clocks go forward 

Unhealthy sleep, unhealthy heart. Simple and true.

As we approach midlife, and our body and health begin to deteriorate, the impact of insufficient sleep on the cardiovascular system escalates.

Adults aged 45 or older who sleep fewer than six hours a night are 200 per cent more likely to have a heart attack or stroke during their lifetime, compared with those sleeping seven to eight hours a night.

This finding emphasises how important it is to prioritise sleep in midlife — which is unfortunately the time when family and professional circumstances encourage us to do the exact opposite.

It doesn’t take much sleep deprivation to impact your cardiovascular system. One night of modest sleep reduction — even just one or two hours — will promptly speed the contracting rate of a person’s heart, hour upon hour.

This also causes a significant increase in systolic blood pressure, which puts greater strain on the heart and blood vessels.

You will find no solace in the fact that the experiments that confirmed this were conducted in young, fit individuals, all of whom started out with an otherwise healthy cardiovascular system just hours before. Such physical fitness proves no match for a short night of sleep and affords no resistance.

Beyond accelerating your heart rate and increasing your blood pressure, a lack of sleep damages those strained blood vessels. The coronary arteries, which supply the heart with blood, are especially affected. These need to be clean and stay open wide at all times.

If these passageways are narrowed or blocked, your heart can suffer a comprehensive — and often fatal — attack caused by blood oxygen starvation.

One cause of a coronary artery blockage is atherosclerosis, where hardened plaques which contain calcium deposits build up in the arteries.

Researchers at the University of Chicago studied almost 500 healthy midlife adults, none of whom had any existing heart disease or signs of atherosclerosis. They tracked the health of the coronary arteries of these participants for a number of years, all the while assessing their sleep.

Those individuals who were getting just five to six hours of sleep each night, or less, were 200 to 300 per cent more likely to suffer calcification of their coronary arteries over the next five years. This was relative to those who slept for seven to eight hours.

When communicating science to the public in lectures or writing, I’m always wary of bombarding an audience with never-ending mortality and morbidity statistics, lest they themselves lose the will to live.

It is hard not to do this with so many compelling studies in the field of sleep deprivation. Often, however, a single astonishing result is all that people need to understand the issue.

For cardiovascular health, I believe that finding comes from a ‘global experiment’ in which more than 1.5 billion people are forced to reduce the amount of sleep they get by one hour, for a single night each year.

It is very likely that you have been part of this experiment, otherwise known as daylight saving time.

In the Northern Hemisphere, the switch to daylight saving time in March results in most people losing an hour of sleep. Researchers have tabulated millions of hospital records and discovered that this seemingly trivial sleep reduction comes with a frightening spike in heart attacks the following day.

It works both ways. In the autumn, rates of heart attacks plummet the day after the clocks go back, when we gain an hour of sleep.

A similar rise-and-fall relationship can be seen with the suicide rate, as well as the number of traffic accidents.

This proves that the brain, by way of attention lapses, microsleeps and emotional instability, is just as sensitive as the heart to very small disruptions in sleep.

Most people think nothing of losing an hour of sleep for one night, believing it to be inconsequential. It is anything but. 

Everyone was then given a flu jab. In the days afterwards, researchers took blood samples to determine how effective each person was in generating an antibody response, working out whether or not the vaccination was a success.

Those participants who had the greater amount of sleep generated a powerful antibody reaction. This reflects a very robust, healthy immune system.

In contrast, those in the sleep-restricted group produced less than 50 per cent of the immune reaction their well-slept counterparts did.

Similar consequences of too little sleep have since been reported for the hepatitis A and B vaccines.

Perhaps the sleep-deprived group could still go on to produce a more resilient immune reaction if only they were given enough sleep recovery time? It’s a nice idea, but a false one.

Even if someone is allowed two or three weeks of recovery sleep to get over the effects of one week of short sleeping, they never go on to develop a full immune reaction to the flu jab.

In fact, certain immune cells are still diminished up to a year after just a minor, short amount of sleep restriction.

It doesn’t require many nights of short sleeping before the body is rendered immunologically weak — and here the issue of cancer becomes relevant.

Natural killer cells are an elite and powerful squadron within the ranks of your immune system. Think of them as the secret service agents of your body, whose job it is to identify dangerous foreign elements and eliminate them — 007 types, if you will.

One such foreign entity that natural killer cells will target are cancerous tumour cells. Natural killer cells will effectively punch a hole in the outer surface of these cells and inject a protein that can destroy the malignancy.

What you want, therefore, is a virile set of these James Bond-like immune cells at all times. And that is precisely what you don’t have when you sleep too little.

Dr Michael Irwin, at the University of California, Los Angeles, has carried out landmark studies revealing just how quickly and comprehensively a brief spell of short sleep can affect your cancer-fighting immune cells.

Examining healthy young men, Dr Irwin demonstrated that a single night of four hours of sleep — such as going to bed at 3am and waking up at 7am — swept away 70 per cent of the natural killer cells circulating in the immune system, relative to a night of a full eight hours of sleep.

That is a dramatic state of immune deficiency to find yourself facing after essentially one ‘bad night’ of sleep.

You could well imagine the enfeebled state of your cancer-fighting immune armoury after a week of short sleep, let alone months or even years.

Well, we don’t have to imagine. A number of prominent epidemiological studies have reported that night-time shift work, and the disruption to circadian rhythms and sleep that it causes, considerably ups your odds of developing many forms of cancer.

These include breast cancer, prostate cancer, endometrial cancer and colon cancer.

As if that were not enough, a recent study by Dr David Gozal, from the University of Chicago, showed that sleep-deprived mice suffered a 200 per cent increase in the speed and size of cancer growth, relative to those mice who were well-rested.

Worse, when Dr Gozal performed postmortem examinations on the mice, he discovered that the tumours were far more aggressive in those who were sleep-deficient. Their cancer had metastasised, spreading to surrounding organs, tissue and bone.

Indeed, the scientific evidence linking sleep disruption and cancer is now so damning that the World Health Organisation has officially classified night-time shift work as a ‘probable carcinogen’.

  • Adapted from Why We Sleep: The New Science Of Sleep And Dreams by Matthew Walker, published by Penguin Books at £9.99. © Matthew Walker 2018. To buy a copy for £7.99 (20 per cent discount), call 0844 571 0640 or go to mailshop.co.uk/books. P&P is free on orders over £15. Offer valid until August 11, 2018.

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