In his popular and oft-cited book, Why We Sleep: The New Science of Sleep and Dreams, professor Matthew Walker, Ph.D., founder and director of the University of California Berkeley's Center for Human Sleep Science, details many supposed benefits of longer sleep.

I've frequently referenced Walkers book in a number of my previous articles about sleep, which is why I became more than a little curious when I came across the work of Alexey Guzey.1

He claims to have spent more than 130 hours over the course of two months investigating the claims presented in only one chapter of Walker's book, Why We Sleep, coming to the conclusion that the chapter, and likely the book, is "riddled with scientific and factual errors."
"Continuing at the same pace, it would take me more than 3,000 hours to check the entire book. 3,000 hours is the equivalent of 75 weeks or 1.4 years of full-time work," Guzey writes.2 "I hope that going through one full chapter, rather than cherry-picking stuff from across the book, demonstrated the density of errors in the book."
While I do not have the kind of time required to duplicate Guzey's double-checking of Walker's work, I decided to present some of Guzey's findings here so that you can review them for yourself.

Certainly, I believe optimizing your sleep is crucial for mental and physiological health. The evidence for this is overwhelming. The question is how much sleep one actually needs, and whether more sleep equates to better health and increased longevity. Guzey's findings contradict some of these assumptions.

Will Longer Sleep Increase Your Life Span?

On page 4 in Why We Sleep, Walker states — without corroborating references — that "the shorter your sleep, the shorter your life span." According to Guzey, this claim does not conform to findings in the medical literature, most of which show a U-shaped relationship between the length of sleep and longevity.

In other words, life span decreases at both ends of the spectrum, and Guzey cites three studies3,4,5 demonstrating this nonlinear association. He points out that "The studies typically find that people who sleep 7 hours have the highest longevity."

Figure 1 below is from a 2016 meta-analysis6 of 35 studies, published in Scientific Reports, which illustrates the nonlinear impact of sleep duration on all-cause mortality risk.

The solid black line in the middle is the estimated relative risk, while the two dashed black lines on either side represent the 95% confidence interval. Guzey added the red dashed lines to highlight the lowest risk zone.

health risk sleep duration graph
Based on this meta-analysis, the lowest all-cause mortality was just short of seven hours of sleep per night, with rates rising sharply after seven hours. The estimated relative risk (solid black line) actually appears to be near-identical at four hours and eight hours of sleep.

Can Lack of Sleep Kill You?

Interestingly, while it's generally accepted that all animals sleep — which would support the idea that there's a definitive biological need for it — this turns out to be false as well.

Guzey points out that Walker contradicts himself on this issue in Why We Sleep — in one instance stating that "Every species studied to date sleeps," while citing the Encyclopedia of Sleep, a 2,736-page book that states "many species reduce sleep for long periods of time ... and that others do not sleep at all, in the way sleep is conventionally defined."7

Guzey also has a problem with Walker's claim that fatal familial insomnia (FFI) is evidence that lack of sleep can kill you. FFI is basically a genetically inherited prion disease, causing severe brain damage and organ failure.

While chronic insomnia is a symptom, "It is reckless to claim that people with FFI die because of lack of sleep, given the amount of damage across the brain that accumulates in the course of the disease," Guzey says.8

What's more, the Encyclopedia of Sleep Walker cites actually states "There is little evidence that sleep induced by sedation can greatly extend life in FFI patients," which would suggest the lack of sleep has little to nothing to do with the demise of these patients.

Other research9 also notes that, ultimately, neurodegeneration appears to be the cause of death among FFI patients. So, on the whole, it's probably not a good idea to draw any particular conclusions about the importance of sleep based on this disease.

Will Short Sleep Double Your Risk of Cancer?

In his book, Walker also claims that "Routinely sleeping less than six or seven hours a night demolishes your immune system, more than doubling your risk of cancer."

Guzey points out that this claim is, again, unsupported. Walker actually doesn't cite any research for this statement, and "There do not appear to exist any experimental studies or studies that would reasonably be able to establish causality, that would support this claim," Guzey says.10

Meanwhile, epidemiological evidence — which cannot show or establish causality — seem to refute the idea that sleeping less than six hours a night might significantly increase your cancer risk.

Guzey cites a 2018 systematic review11 of 65 studies published in BMC Cancer — which included a total of 1,550,524 participants and 86,201 instances of cancer — that suggest the cancer risk remains near-identical for those sleeping between 5.5 hours and eight hours per night.

Beyond eight hours a night, the cancer risk slowly increases, but not by much. Ditto for less than 5.5 hours of sleep. On his website,12 Guzey presents the following graph from that BMC Cancer review13 (again with the red dotted lines added for clarity).

cancer and sleep duration graph
However, Guzey's analysis also leaves some things out, as the BMC Cancer study14 he quotes does, in fact, state that subgroup analysis revealed that "short sleep duration increased cancer risk in Asians" by 36%, while "long sleep duration increased the risk of colorectal cancer."

People Consistently Overestimate Their Sleep Time

The problem with epidemiological studies is the lack of verifiable accuracy, since most use self-reported data, and people can easily misjudge how much sleep they actually get on a regular basis. Guzey cites a study15 published in the journal Epidemiology, which found that people consistently over-reported their subjective sleep time. According to the authors:16
"Average measured sleep was 6 hours, whereas the average from subjective reports was 6.8 hours. Subjective reports increased on average by 34 minutes for each additional hour of measured sleep.

Overall, the correlation between reported and measured sleep duration was 0.47 ... the average difference at the mean of 6 hours measured sleep was 0.80 hours (48 minutes)."
For this reason, it's unclear just how meaningful epidemiological findings really are. As noted by the authors of this Epidemiology study, "The true associations between sleep duration and health may differ from previously reported associations between self-reported sleep and health."17

So, while these kinds of studies can offer some general clues about correlation, one should remember that results are easily skewed, and when it comes to sleep, that people across the board tend to over-report the amount of sleep they believe they're getting.

Guzey applies this finding to Figure 1 (above) from the 2016 meta-analysis18 that found those with the lowest mortality slept right around seven hours, pointing out that if you take this over-reporting tendency into account, the lowest mortality would actually end up being around six hours.19

Is Sleep Deprivation at Epidemic Proportions?

Next, Guzey tackles Walker's claim that "The World Health Organization (WHO) has now declared a sleep loss epidemic throughout industrialized nations," on page 4 of "Why We Sleep."

As far as Guzey could determine, WHO has never made such a declaration, and the source for Walker's claim is the National Geographic documentary Sleepless in America, which also does not mention WHO. What's more, most of the sources making this claim actually use Walker as their source, Guzey points out. My own online search, while not comprehensive, revealed the following:
- A 1997 article in JAMA titled "World Health Organization Targets Insomnia" discusses the Worldwide Project on Sleep and Health, "a 20-year global mission to improve the treatment of sleep disorders under the aegis of the World Health Organization (WHO) and World Federation of Sleep Research Societies (WFSRS)."

- The SAGE-INDEPTH study20 is a multi-center health and well-being survey led by WHO, which includes questions about sleep.21

A 2012 report22 in the journal Sleep, titled "Sleep Problems: An Emerging Global Epidemic? Findings From the INDEPTH WHO-SAGE Study Among More Than 40,000 Older Adults From 8 Countries Across Africa and Asia," points out that "A large number of older adults in low-income settings are currently experiencing sleep problems, which emphasizes the global dimension of this emerging public health issue."

While the inclusion of a question mark in that title prevents it from being a direct statement, this paper could potentially be a source for Walker's statement that WHO "has now declared a sleep loss epidemic." At bare minimum, WHO's sleep data are the basis for the researcher's suggestion that sleep problems could be a global epidemic.

- A 2014 paper23 in PLOS ONE also covered the results of WHO's sleep data, stating that "the prevalence of severe/extreme sleep problems to be as high as 43.9% among Bangladeshi women, with prevalence figures higher than 25% observed also in Vietnam and South Africa, suggesting that there may be an under-recognized emerging global epidemic of sleep problems."
So, that's two papers suggesting WHO's data point to a global epidemic of sleep problems. What's still missing is a reference for the "throughout industrialized nations" part of Walker's statement, as Africa and Asia are unlikely to represent industrialized nations.

As for WHO recommendations on sleep duration, I've not found any, either. A WHO technical meeting on sleep and health document24 from January 2004 discusses the health ramifications of insufficient sleep, but does not include any specific recommendations on adult sleep duration.

WHO does, however, have guidelines for sleep duration for children under the age of 5. In its 2019 report,25 "Guidelines on Physical Activity, Sedentary Behavior and Sleep for Children Under 5 Years of Age," WHO recommends infants aged 4 to 11 months get 12 to 16 hours of good quality sleep including naps; infants aged 1 to 2 should get 11 to 14 hours; and children aged 3 to 4 need 10 to 13 hours.

Insufficient Sleep Is Likely Rampant

All of that said, other researchers have stressed there's a global sleep loss epidemic afoot, and that public health could indeed be improved were people to sleep a bit more. For example, a March 2019 review published in the journal Healthcare, states:26
"Insufficient sleep is a pervasive and prominent problem in the modern 24-h society. A considerable body of evidence suggests that insufficient sleep causes hosts of adverse medical and mental dysfunctions. An extensive literature search was done in all the major databases for 'insufficient sleep' and 'public health implications' in this review.

Globally, insufficient sleep is prevalent across various age groups, considered to be a public health epidemic that is often unrecognized, under-reported, and that has rather high economic costs. This paper addresses a brief overview on insufficient sleep, causes, and consequences, and how it adds to the existing burden of diseases.

Insufficient sleep leads to the derailment of body systems, leading to increased incidences of cardiovascular morbidity, increased chances of diabetes mellitus, obesity, derailment of cognitive functions, vehicular accidents, and increased accidents at workplaces. The increased usage of smart phones and electronic devices is worsening the epidemic."
Do a Majority of Adults Not Get Recommended Amount of Sleep?

Walker's claim that two-thirds of adults in developed nations fail to get the recommended amount of sleep appears to have a similar sourcing problem. Guzey writes:27
"Suppose that you recommend that adults sleep 7-9 hours per night. Then, someone learns28 (a) that roughly 40% of people sleep less than 7 hours, roughly 25% sleep 7 hours, and roughly 35% sleep 8 hours or more, meaning that a bit over one-third of people sleep less than you recommend

Then they look at your recommendation and say that you recommended an average of 8 hours of sleep per night ... Then they say that you recommended 8 hours of sleep per night, Then they say that two-thirds of people sleep less than the 8 hours you recommended

Would this be a fair representation of your position and of the data or would this be misleading? This is literally what Walker does in his book. On page 3, in the very first paragraph of Chapter 1, Walker writes: 'Two-thirds of adults throughout all developed nations fail to obtain the recommended eight hours of nightly sleep.'

In the footnote to this sentence he writes: 'The World Health Organization and the National Sleep Foundation both stipulate an average of eight hours of sleep per night for adults.'

Here are the National Sleep Foundation's sleep recommendations29 announced in 2015: 'Adults (26-64): Sleep range did not change and remains 7-9 hours.' Here are the World Health Organization's sleep recommendations:

‎The quote is empty because the WHO does not stipulate how much an adult should sleep anywhere. I don't know where Walker got this information."
What Harm Can Walker's Book Do?

Considering the importance of sleep for biological and psychological functioning, which I cover in "Sleep — Why You Need It and 50 Ways to Improve It," one might argue that Walker's book can do no harm by, perhaps, overstating the risks and encouraging eight hours of sleep. Guzey disagrees, saying, in part:30
"... imagine that a 20-year-old who naturally needs to sleep for 7 hours a night, reads 'Why We Sleep,' gets scared, and decides to spend the full 8 hours in bed every day.

Then, assuming that they live until 75, they will waste more than 20,000 hours or more than 2 years of their life, with uncertain long-term side-effects. Finally, to be less speculative, here's an email from a sleep coach Martin Reed I got after the publication of this essay:

'Hello Alexey

I wanted to drop you a line to thank you for all the time and effort involved in debunking Matthew Walker's book. As someone who works with individuals with insomnia on a daily basis, I know from firsthand experience the harm that Walker's book is causing.

I have many stories of people who slept well on less than eight hours of sleep, read Walker's book, tried to get more sleep and this led to more time awake, frustration, worry, sleep-related anxiety, and insomnia' ...

A sleep physician Daniel Erichsen writes: 'Dear Alexey ... I can't thank you enough. I'm a sleep doctor in Oregon and have seen many many patients who have developed severe sleep anxiety and insomnia. Two friends in the sleep field and myself weekly have talked about people that slept well until reading this book.'"
Unsourced, Unsupported Data Entering Scientific Literature

Guzey also points out the problem with Walker's book becoming a source in itself, given it doesn't appear to stand on solid scientific ground.

"This is how academic urban legends are created," he says, citing academic papers that use "Why We Sleep" as sources for statements such as "All known forms of animal life must sleep," and "the World Health Organization has pointed to a 'global epidemic of sleeplessness' with roughly two-thirds of adults sleeping less than 8 h a night" — statements of "facts" that Guzey show are either false or unsupported.

Perhaps even more problematic, Walker sources his own book in his own academic papers; one published in The Lancet in 2018 and one in Neuron in 2019.

As noted by Guzey, "the quoted statistics from 'Why We Sleep' are unsourced and we have no way to see how or where Walker got these numbers. Via The Lancet and Neuron, they have now entered the scientific literature, while lacking a primary source."

To Sleep More or Not Sleep More, That Is the Question

At the end of the day, how much sleep you need is quite personal, and you are probably the best person to assess whether your sleep habits are harmful or helpful. How do you feel when you wake up? How are your energy levels while driving to and from work? When do you get sleepy?

Sleepiness during the day is a telltale sign of insufficient sleep. Ditto if you're really dragging your heels in the morning and need multiple cups of coffee to get yourself into a productive space. In the evening, do you heed your body's call for sleep, or do you push through and stay awake another hour or more?

The key, I think, is being truly honest with yourself. Do you look, feel and behave your best after five, six, seven or eight hours of sleep? How much sleep do you need to be truly productive the next day? Take into account your mood too. Short-temperedness and getting easily frustrated are other telltale signs you've not slept enough.

That said, guidelines are typically helpful, provided they're based on decent science. In 2015, the National Sleep Foundation updated their sleep guidelines based on a review of more than 300 studies published between 2004 and 2014 that looked at the sleep duration required to maintain health.

As noted earlier, the adult recommendations (ages 26 through 64) remained unchanged at between seven and nine hours, with an average of eight. The recommendation for seniors over 65 is seven to eight hours.

Now, it would take an enormous amount of time to double-check those 300 studies to determine how many were based on self-reported sleep, and thus likely over-estimated the needed amount of sleep to maintain health and prevent disease.

So, your best bet is to simply remember that sleep recommendations are not hard and fast. They're guidelines, and if you're sleeping like a prince or princess on seven hours and feel great, then forcing yourself to sleep more could be a mistake.

If you're not feeling good emotionally or physically, however, or if you're struggling with an acute or chronic health problem, increasing your sleep time may be part of your answer. Experiment and see what makes you feel your best. Chances are, you're not going to put yourself in jeopardy if you're following your body's cues.

- Sources and References