THE ARTICLE · 11 MIN
Why We Sleep became a bestseller (Penguin calls it “the #1 Sunday Times bestseller”) and also drew a detailed published critique. Here are its big ideas in our own words, what the research says about ten of its claims, the criticism and Walker’s replies side by side, and what public-health bodies say about sleep.
About the book
Why We Sleep is by Matthew Walker, a neuroscientist who studies sleep. It came out in 2017: in London from Allen Lane, subtitled The New Science of Sleep and Dreams, and in New York from Scribner, subtitled Unlocking the Power of Sleep and Dreams. At the time, the publisher described him as Director of UC Berkeley’s Sleep and Neuroimaging Lab. His website now lists him as a professor at the University of Texas at Dallas, after 17 years at UC Berkeley.
It is popular science with a practical appendix. Part 1 explains what sleep is, Part 2 why the brain and body need it, Part 3 how and why we dream, and Part 4 covers sleep disorders, sleeping pills versus therapy, and sleep in schools, medicine and workplaces. The appendix gives twelve tips for healthy sleep.
The book is in copyright, so we did not read it in full. We read the library records of the 2017 US and UK editions, short passages through the Internet Archive’s search-inside tool, the published critique of its first chapter, Walker’s replies, and the studies listed below. This page describes a book and the research about it. It is not medical advice.
The big ideas
1. Two forces decide when you sleep
Walker explains sleep timing with two forces. One is a body clock that runs on a roughly twenty-four-hour cycle. The other is “sleep pressure”: a chemical called adenosine builds up the longer you are awake, and with it the urge to sleep. The second chapter covers caffeine and jet lag, and how each one affects these forces.
2. Sleep runs in cycles
Through the night, the brain moves back and forth between NREM and REM sleep about every ninety minutes. The book argues that this back-and-forth is needed to “remodel and update” the brain’s circuits.
3. Sleep before and after learning
The chapter on the brain describes two benefits for memory. Sleep before learning, in Walker’s words, “refreshes our ability to initially make new memories”; in a 2007 study from his lab, people kept awake for a night remembered less of what they learned the next day. The second benefit, in the chapter’s words, “comes after learning”.
4. Short sleep and the body
The eighth chapter, “Cancer, Heart Attacks, and a Shorter Life”, links short sleep with illness across the body. The first chapter puts the case at its strongest: “the shorter your sleep, the shorter your life span”.
5. Dreams have their own work to do
Part 3 argues that REM-sleep dreaming serves real needs; the book speaks of “what vital needs it supports”. One chapter is titled “Dreaming as Overnight Therapy” and another is about dreams and creativity.
6. Most adults sleep too little
The book opens by saying that two-thirds of adults in developed nations do not get the recommended eight hours. It quotes Dr Thomas Roth of the Henry Ford Hospital in Detroit: the share of people who can survive on five hours or less without any impairment, “rounded to a whole number, is zero”. Walker adds that a very small group seems to manage on six hours with little impairment, linked to a variant of a gene called BHLHE41.
7. A society that undervalues sleep
The last part turns to schools, medicine and work. Walker criticises early school start times for teenagers, and describes a psychological treatment for insomnia, CBT-I, as “rapidly being embraced by the medical community as the first-line treatment”.
What holds up
Much of the book’s core is supported. Lab studies find that steady short sleep impairs attention, and sleep helps memory, though by less than earlier work suggested. Short sleep is also linked with weaker immune responses. Several of its most quoted lines go further than the evidence: the WHO line, the cancer line, and the line on lifespan, which leaves out that long sleep is linked with higher death rates too. The rows below give each claim, a verdict and the sources.
The criticism, and Walker’s replies
In November 2019 Alexey Guzey published an essay titled “Matthew Walker’s ‘Why We Sleep’ Is Riddled with Scientific and Factual Errors”. He checked the first chapter only, which he said makes up “less than 4% of the book”, and argued that the book “consistently overstates the problem of lack of sleep”. Among his points were the lifespan line, the cancer line, the WHO line, and the eight-hour recommendation. In an appendix he showed that the book’s chart of a study on young athletes had left out the five-hour bar, whose injury rate was lower than the six-hour bar’s.
The statistician Andrew Gelman discussed the essay on his blog that month. He wrote that he had not read the book and did not know sleep research, but found Guzey’s arguments persuasive, and that “the ball is totally in Walker’s court”.
Walker replied in December 2019, in a long post answering readers’ questions about the book. On the WHO line, he wrote that the CDC, not the WHO, had called insufficient sleep a public health epidemic, and that the misattribution would be corrected. On cancer, he said short sleep is linked with some cancers but not all, and that this “will be reflected in future editions”. On the chart, he said the study’s authors had told him the five-hour group was small, and that later editions would show the pooled comparison. On lifespan, he quoted the book’s later chapter on the backward-J curve and argued that a 2016 meta-analysis cited against the book measured sleep only once, sometimes more than 20 years before death, and so could not capture changes in sleep. He also said a note would be added for readers who feel anxious about their sleep. In an update to the post, he wrote that the corrections had been published in a second edition; we have not seen that edition.
The lifespan curve, in more detail
The lifespan claim shows why both sides matter. The 2010 meta-analysis by Cappuccio and colleagues, covering 16 studies, found that both short and long sleep predicted a higher risk of death. These studies follow people over years and ask them how long they sleep, so they show links, not causes; illness, for example, may lengthen sleep, rather than long sleep causing illness. Walker’s reply points to a study of 21,268 Finnish twins, asked about their sleep in 1975 and again in 1981, that linked less than seven hours with a higher risk of death; the same study found a higher risk with long sleep too. The expert panel that set the seven-hour minimum placed no upper limit, and Nathaniel Watson, who moderated it, said that long sleep is “more likely to reflect chronic illness than to cause it”, and that “few experimental laboratory studies have examined the health effects of long sleep duration”. So the evidence leans one way for short sleep and another for long sleep. Short sleep has both the population links and lab experiments showing short-term harm, such as the 2003 experiment in which six-hour nights impaired performance. Long sleep has the links but few experiments, and the panel’s moderator reads it as more often a sign of illness. Neither end is settled by the population studies alone.
How to use it
These draw on the parts of the book that hold up and on what public-health bodies say. They describe; what fits your life is your call.
- Using seven hours as the benchmark, not eight. The American Academy of Sleep Medicine and the Sleep Research Society recommend that adults obtain seven or more hours of sleep per night, and the CDC uses the same minimum for adults aged 18 to 60. The National Sleep Foundation’s range for adults is 7 to 9 hours.
- Keeping a regular schedule. The first of the book’s twelve tips is to stick to a sleep schedule, and the NHS lists waking up and getting out of bed at the same time every day among the habits that help with insomnia.
- Timing caffeine and alcohol. The NHS insomnia page lists avoiding alcohol, tea or coffee for at least six hours before bed. The book’s second chapter covers caffeine and how it acts on sleep pressure.
- Knowing the first-line treatment for insomnia. The American College of Physicians recommends cognitive behavioural therapy for insomnia (CBT-I) as the initial treatment for adults with chronic insomnia, the same therapy the book describes.
- Not keeping a frightening score. Walker’s own reply says that some readers who struggle with sleep may feel more anxious after reading about the risks, and that a cautionary note would be added. The large studies behind the scariest numbers show links across populations, not a verdict on any one person’s night.
Anyone worried about their sleep, or with symptoms such as loud snoring, gasping or choking in the night, or feeling very tired during the day, should speak to a doctor. The NHS, for example, says to see a GP about the main symptoms of sleep apnoea, such as breathing that stops and starts during sleep.
Who it’s for, and who can skip it
It suits readers who want a lively tour of sleep science, from body clocks to dreams, written by a researcher in the field. Its strongest parts, on what sleep does for attention and memory, match the research we checked. Readers who are already anxious about their sleep may find its framing heavy; Walker himself has said so. Anyone reading it for its numbers can read it next to this page and the critique, since several of the most quoted figures are stronger in the book than in the studies behind them.
If you liked this
- How to read a study without a science degree: the questions that separate a link from a cause, which is at the heart of the lifespan debate.
- Numbers that mislead, including relative change without the base and bar charts that do not start at zero.
- The Power of Habit, summarised, for the research on how routines form.
- Getting Things Done, summarised, which covers a study on writing a to-do list before bed.
Sources
- M. Walker, Why We Sleep: Unlocking the Power of Sleep and Dreams (Scribner, 2017) and Why We Sleep: The New Science of Sleep and Dreams (Allen Lane, 2017): library records (US edition, UK edition) and short passages via the Internet Archive’s search-inside tool; Penguin’s paperback page; the author’s website.
- A. Guzey, “Matthew Walker’s ‘Why We Sleep’ Is Riddled with Scientific and Factual Errors” (created 15 November 2019).
- A. Gelman, “Is Matthew Walker’s ‘Why We Sleep’ Riddled with Scientific and Factual Errors?” (18 November 2019), Statistical Modeling, Causal Inference, and Social Science (archived copy).
- M. Walker, “Why We Sleep: Responses to questions from readers” (19 December 2019, updated February 2021).
- F. P. Cappuccio, L. D’Elia, P. Strazzullo and M. A. Miller, “Sleep duration and all-cause mortality”, Sleep 33 (2010); Z. Ungvari et al., “Imbalanced sleep increases mortality risk by 14-34%: a meta-analysis”, GeroScience (2025); C. Hublin, M. Partinen, M. Koskenvuo and J. Kaprio, “Sleep and mortality: a population-based 22-year follow-up study”, Sleep 30 (2007).
- Y. Chen et al., “Sleep duration and the risk of cancer”, BMC Cancer 18 (2018); A. T. Y. Wong et al., “Sleep duration and breast cancer incidence”, Sleep 44 (2021).
- A. A. Prather, D. Janicki-Deverts, M. H. Hall and S. Cohen, “Behaviorally assessed sleep and susceptibility to the common cold”, Sleep 38 (2015); K. Spiegel et al., “A meta-analysis of the associations between insufficient sleep duration and antibody response to vaccination”, Current Biology 33 (2023).
- J. M. Siegel, “Do all animals sleep?”, Trends in Neurosciences 31 (2008); C. Cirelli and G. Tononi, “Is sleep essential?”, PLoS Biology 6 (2008); R. D. Nath et al., “The jellyfish Cassiopea exhibits a sleep-like state”, Current Biology 27 (2017).
- A. Sandhu, M. Seth and H. S. Gurm, “Daylight savings time and myocardial infarction”, Open Heart (2014); A. Hurst, P. Morfeld, P. Lewis and T. C. Erren, “Daylight saving time transitions and risk of heart attack”, Deutsches Ärzteblatt International (2024); L. Derks et al., “Daylight saving time does not seem to be associated with number of percutaneous coronary interventions for acute myocardial infarction in the Netherlands”, Netherlands Heart Journal (2021); J. A. Rymer et al., “Daylight savings time and acute myocardial infarction”, JAMA Network Open (2025).
- H. P. A. Van Dongen, G. Maislin, J. M. Mullington and D. F. Dinges, “The cumulative cost of additional wakefulness”, Sleep 26 (2003); H. P. A. Van Dongen, M. D. Baynard, G. Maislin and D. F. Dinges, “Systematic interindividual differences in neurobehavioral impairment from sleep loss”, Sleep 27 (2004); Y. He et al., “The transcriptional repressor DEC2 regulates sleep length in mammals”, Science 325 (2009); R. Pellegrino et al., “A novel BHLHE41 variant is associated with short sleep and resistance to sleep deprivation in humans”, Sleep 37 (2014); M. N. Weedon et al., “The impact of Mendelian sleep and circadian genetic variants in a population setting”, PLoS Genetics 18 (2022).
- B. Rasch and J. Born, “About sleep’s role in memory”, Physiological Reviews 93 (2013); M. J. Cordi and B. Rasch, “How robust are sleep-mediated memory benefits?”, Current Opinion in Neurobiology 64 (2021); S. Berres and E. Erdfelder, “The sleep benefit in episodic memory”, Psychological Bulletin 147 (2021); S.-S. Yoo, P. T. Hu, N. Gujar, F. A. Jolesz and M. P. Walker, “A deficit in the ability to form new human memories without sleep”, Nature Neuroscience 10 (2007).
- M. D. Milewski et al., “Chronic lack of sleep is associated with increased sports injuries in adolescent athletes”, Journal of Pediatric Orthopaedics 34 (2014); B. Gao, S. Dwivedi, M. D. Milewski and A. I. Cruz, “Lack of sleep and sports injuries in adolescents”, Journal of Pediatric Orthopaedics (2019).
- Y. S. Bin, N. S. Marshall and N. Glozier, “Sleeping at the limits”, American Journal of Epidemiology (2013).
- CDC feature page “Insufficient Sleep Is a Public Health Epidemic” (archived March 2011); the same page as “Insufficient Sleep Is a Public Health Problem” (archived September 2015).
- Y. Liu et al., “Prevalence of healthy sleep duration among adults — United States, 2014”, MMWR 65 (2016).
- M. Hirshkowitz et al., “National Sleep Foundation’s sleep time duration recommendations”, Sleep Health (2015); American Academy of Sleep Medicine, “Seven or more hours of sleep per night: a health necessity for adults” (press release, 2015).
- A. Qaseem et al., “Management of chronic insomnia disorder in adults”, Annals of Internal Medicine (2016).
- NHS, “Insomnia” and “Sleep apnoea”.
Checked October 2026. What we read: the library records and short search-inside passages of the 2017 Scribner and Allen Lane editions (not the whole book, and not any later edition), Guzey’s essay, Gelman’s November 2019 post (via an archived copy), Walker’s 2019 reply, the public-health pages above, and the studies (abstracts, plus the full text of the 2008 review by Cirelli and Tononi).
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