“The myth of eight hours”: why you may not need as much sleep as you thought

“The myth of eight hours”: why you may not need as much sleep as you thought

Daniel Lieberman, author of new book Exercised: The Science of Physical Activity, Rest and Health, explains the science of slumber and why you needn’t stress about getting exactly eight hours.

Daniel Lieberman

Until there is a vaccine for Covid-19, combatting the pandemic will necessarily rely on masks, social distancing and handwashing to prevent the virus from spreading along with therapeutics to aid those who get seriously sick. But there is another simple thing we all can and should do: boost our immune systems. And of the various ways to boost immunity, the simplest is to get enough sleep.

Unfortunately, to numerous hard-working, stressed-out folks, getting enough sleep is a luxury, and to some like Virginia Woolf, not to mention many college students, sleep is a “deplorable curtailment of the joy of life”. Or as Margaret Thatcher contemptuously remarked, “Sleep is for wimps.” You may agree, but chronic sleep deprivation elevates the body’s sympathetic (fight and flight) nervous system, which in turn depresses the immune system. Numerous studies show that people who get too little sleep are more vulnerable to infectious diseases, including respiratory tract infections.

But how much sleep is enough? Most of us learn that eight hours is normal and recommended. Eight hours is axiomatic, virtuous, and simple. Yet as Albert Einstein warned, “Everything should be made as simple as possible, but no simpler.” Exhortations to sleep eight nightly hours not only oversimplify how much most of us need, they can also potentially make us more stressed – and hence, less likely to get enough sleep.

Isn’t that heresy? It’s commonly believed that people used to sleep more before lightbulbs, TVs, cell phones and other modern inventions brutishly assaulted our sleep habits. Thomas Edison proudly nicknamed his engineers “the insomnia squad”. The supposed result is a worldwide epidemic of sleep deprivation that apparently afflicts one in three people in nations like the United States. In turn, lack of sleep promotes obesity and other chronic conditions like heart disease and type 2 diabetes. Since stress is the chief enemy of sleep, a vicious cycle can potentially ensue for people who become anxious about their sleep. Such anxieties are profitable for the multi-billion-dollar Sleep Industrial Complex eager to sell us products, including harmful drugs, to help us sleep better.

Adequate sleep is profoundly important for health, and I do not want to trivialise the real and serious problems of those who cannot or do not get enough sleep. But it’s not only an oversimplification, it also doesn’t stand up to the evidence.

Let’s start with how much sleep people actually get. When asked, most adults in places like the United States report sleeping seven to seven-and-a-half hours per night, and one in three say they regularly get less than six hours. Self-reported estimates of sleep, however, are infamously unreliable. How can you possibly judge how long you were unconscious last night? New sensors that objectively measure sleep indicate that the average Western adult sleeps between six and seven hours a night, with variations by gender and race as well as by season. One possible interpretation of these data is that most of us get less sleep than our ancient forebears supposedly enjoyed.

But that’s wrong, too. An eye-opening study by UCLA researcher Jerome Siegel and colleagues affixed wearable sensors to hunter-gatherers from Tanzania, the Kalahari Desert, and the Amazon. None of these populations have electric lights, let alone clocks or internet access. Yet they mostly slept less than eight hours. In warmer months, the foragers slept on average 5.7 to 6.5 hours a night, and during colder months they slept on average 6.6 to 7.1 hours. They also rarely napped. Studies that monitored Amish farmers in the US who shun electricity as well as other non-Western populations such as rural farmers in Haiti and Madagascar report similar average sleep durations, about 6.5 to 7.0 hours a night. Thus, contrary to consensus, non-industrial populations don’t sleep more than people in countries like the US and the UK. What’s more, there is no reliable evidence that average sleep duration in the Western world has decreased in the last 50 years.

If you are reading this sceptically, you might be thinking that just because non-industrial people typically sleep less than eight hours doesn’t mean their habits are healthy. Yet, hunter-gatherers who survive childhood typically live to their seventies without getting any of the chronic conditions associated with sleep deprivation such as obesity, heart disease, and diabetes.

An eye-opening study affixed wearable sensors to populations [with no] electric lights, clocks; they slept on average 5.7 to 6.5 hours

Even more startling is a massive 2002 study by Daniel Kripke and colleagues that examined the health records and sleep patterns of more than one million Americans. Those who slept eight hours a night had 12 percent higher death rates than those who slept six-and-a-half to seven-and-a-half hours, while heavy sleepers who reported more than eight-and-a-half hours and light sleepers who reported less than four hours had 15 percent higher death rates. Since then, numerous studies using more data and sophisticated methods to correct for factors like age, illness, and income have confirmed that people who sleep about seven hours have a higher probability of living longer than those who sleep more or less.

Keep in mind these studies report averages, standard deviations and probabilities. Each of us is different, and while seven hours may be average and optimal for the population as a whole, there is considerable variation, which means that plenty of people likely need and benefit from more or less than seven hours.

Someday, Covid-19 will be a dim, bygone nightmare, but right now the pandemic remains a useful wakeup call to remind us that good sleep hygiene can help boost our immune systems, as well as help us think and cope. If you are worried about your sleep, relax, and ask yourself five questions:

    Are you satisfied with your sleep?
    Do you stay awake all day without dozing?
    Are you asleep between 2:00 and 4:00 a.m.?
    Do you spend less than 30 minutes awake at night?
    Do you get between six and eight hours of sleep?

If your answers are “usually or always,” then sleep contentedly. If not, the two best-studied, safe, and proven approaches to help you are cognitive behavioural therapy and getting regular exercise. But whatever you do, don’t stress about the myth of eight hours.

What did you think of this article? Let us know at editor@penguinrandomhouse.co.uk for a chance to appear in our reader’s letter page.

Illustration: Ryan MacEachern/Penguin

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