Why We Sleep

Why We Sleep
Author: Matthew Walker


The author of this book set out to redress the problem of ‘society’s apathy toward sleep’, a physiological state he refers to as ‘one of the last great biological mysteries‘. In his masterful elucidation, he unveiled a bewildering array of the critical functions sleep subserves in maintaining both physical and psychological health. In this regard, the book is a masterclass in appreciating the regulatory functions of sleep over almost every body system and organ, from immune defensive functions against infections to cancer prevention. Even more fascinating are the book’s themes around the less obvious roles of sleep in such mechanisms as learning, memory, problem-solving, decision-making, creativity and motor skills. The book’s wide remit is reflected in the diversity of subjects it covered, from the relative merits of sleeping pills and cognitive behaviour therapy, to the deleterious effects of daylight saving and shift work. The book’s key message, reflecting the importance of the different themes it covers, is that ‘sleep is the single most effective thing we can do to reset our brain and body health each day’ (pages 5-8, 22-28, 124-129 and 169).


Sleeping gypsy. Niall Kennedy on Flickr. https://www.flickr.com/photos/niallkennedy/54595238


The most captivating theme of the book is its discussion of the two phases of sleep, rapid and non-rapid eye movement (REM and NREM), and their defining features. The book recounted the thrilling history of their discovery, an achievement that the author depicted as ‘arguably the most important discovery in all of sleep research’. Crediting this trailblazing breakthrough to Eugene Aserinsky and Nathaniel Kleitman, the author narrated how the two discovered the 90 minute sleep cycles that REM and NREM phases go through several times a night, and how each phase subserves contrasting but equally essential functions. For example, the author pointed out that REM sleep, also known as paradoxical sleep because of its desynchronised wave pattern resembling wakefulness, is the time when neural connections are strengthened ‘so as to update our memory networks based on the events of the prior day’. The author also metaphorically portrayed REM sleep as a ‘master piano tuner‘ which ‘readjusts the brain’s emotional instrumentation at night to pitch-perfect precision‘. On the other hand, the author referred to NREM sleep as the time when experiences are transferred from short-term to long-term memory storage, akin to ‘a courier service, transporting memory packets from temporary storage hold… to a more secure, permanent home’ (pages 42-45, 52-54, 74-79, 89-92, 207, 114 and 215).

Sleeping girl. Mike Goad on Flickr. https://www.flickr.com/photos/exit78/28322119191

The key physiological theme of the book is the regulation of sleep, and this narrative was dominated by the role of the circadian rhythm. Referring to this as a ‘powerful sculpting force‘, the author explored its influence over sleep and wakefulness, and its capacity to determine people’s chronotype – whether they are more active in the mornings or in the evenings. The book also chronicled the scientific understanding of the circadian rhythm, recounting how it was first observed by Jean-Jacques d’Ortous de Marian in the plant Mimosa pudica. It was however the story of its discovery in humans that properly defined it, a feat achieved by Nathaniel Kleitman and Bruce Richardson when they stayed for 32 days in the total darkness of Mammoth Cave in Kentucky. The author described how Kleitman and Richardson determined that the human circadian rhythm is 24 hours and 15 minutes long, and how daylight acts as ‘a manipulating finger and thumb on the sides of an imprecise wristwatch’ to reset it back to 24 hours. The author also reviewed how the suprachiasmatic nucleus establishes the circadian rhythm, and how other zeitgebers such as food and exercise also reset the rhythm (pages 13-21).

Sleeping Savoyard Boy. Gandalf’s gallery on Flickr. https://www.flickr.com/photos/gandalfsgallery/5956891683

As expected, dreaming constituted a riveting theme in the book with the author presenting an enlightening perspective on the nature of what is indeed a curious phenomenon. Perhaps the most helpful function of dreams is in enabling creativity, a role the author attributed to its capacity to make ‘connections between distantly related informational elements that are not obvious in the light of the waking day’. The author explained that dreams promote creativity by making the brain ‘biased toward seeking out the most distant, non-obvious links between sets of information’, and by doing away with ‘the hierarchy of logical associative connection‘ that obstruct inspiration. The author illustrated his argument for the creative potential of dreams with the examples of how they enabled Dmitri Mendeleyev to envisage the periodic table of elements, and Otto Loewi to design his chemical neurotransmission experiment. Another pivotal function of dreams the book explored is the regulation of emotions, a role the author portrayed as ‘a red-thread narrative that runs from our waking lives into our dreaming lives’. The author referred to the way dreams offer ‘emotional resolution‘ to distressing experiences by eradicating their ‘painful sting‘, and by ‘divorcing the bitter emotional rind from the information-rich fruit‘ of our daily lives. The importance of this role is most evident when it fails and triggers post traumatic stress disorder (pages 204-214, 220-221 and 226-227).

Sleeping Man. Irina on Flickr. https://www.flickr.com/photos/repolco/49871076668

The consequences of sleep deprivation, a growing concern in modern life, is perhaps the most practical theme the book covered. In a narrative which graphically illustrated that ‘no facet of the human body is spared the crippling, noxious harm of sleep loss‘, the book discussed the serious repercussions of insufficient sleep – defined as less than eight hours of sleep a day. The author, for example, reviewed such worrying aftermaths as fatigue, emotional irrationality, aggression, risk taking, and substance abuse, and such risky outcomes as dangerous ‘lapses in concentration‘, road traffic accidents, and work-related errors – the impact of the latter being worse than the effects of ‘alcohol and drugs combined’. Particularly striking was the risk of developing Alzheimer’s disease, a peril that arises because the glymphatic system that clears ‘metabolic refuse‘ such as amyloid requires the sleep phase to function efficiently. Other ominous consequences of sleep deprivation the book discussed included hypertension, obesity, hormonal and menstrual dysfunction, accelerated ageing, and coronary heart disease. All these explain the author’s dire warning that ‘the shorter you sleep, the shorter your lifespan‘, and that ‘lack of sleep can kill you outright’ (pages 3-5, 133-149 and 164-189).

Insomnia. Gandalf’s gallery on Flickr. https://www.flickr.com/photos/gandalfsgallery/27395721962

The practical recommendations the author made to maximise the efficiency of sleep constitute the most helpful elements of the book. In this regard, the most momentous, and for many, the most life-changing, exhortation he made was for a complete abstinence from alcohol. Apart from causing profound sleep fragmentation, the author argued that this intoxicant also impedes sleep-related memory consolidation, referring to it is ‘one of the most powerful suppressors of REM sleep that we know of’. The book reviewed other practical measures to improve the quality of sleep, and these included avoiding exercise within two hours of bedtime; shunning caffeine in the evening because this stimulant ‘mutes’ the sleep-promoting effect of adenosine – the ‘chemical barometer‘ that keeps account of the time since a person woke up; and turning off blue LED lighting in the evenings because this suppresses melatonin production by deceiving the suprachiasmatic nucleus ‘into believing the sun has not yet set’ (pages 82-85, 271-275, 282-294, 157 and 267-268).

Night and HerDaughter Sleep. Smithsonian American Art Museum on Flickr. https://www.flickr.com/photos/americanartmuseum/3533164449


This is a revealing book about the mysterious activity that is sleep. Written by a leading sleep researcher, the book lucidly explains what really happens when we slumber, just as it strongly warns of the consequences of not having enough sleep. With countless and colourful metaphors, the author illustrated the mechanisms driving sleep, and emphasised the critical complementary roles of its two phases. The author did not delve into the clinical manifestations of many sleep disorders, his exploration limited to somnambulism, narcolepsy, and fatal familial insomnia; this is however perhaps because these are outside the author’s main agenda which is to stress the important physiological functions that sleep carries out. The book’s strengths lie in its detailed and graphic depictions of sleep, its helpful references to the history of sleep medicine, and its far-reaching recommendations on sleep hygiene. These make it a most eloquent advocate for a fundamental element of healthcare.

Overall assessment

The lessons of this book are life-changing, demonstrating as they do, the critical functions played by sleep, and the consequences when these are disrupted. With a focus on the practical implications of what is now known about sleep, the book’s value goes beyond encouraging a good night sleep, and extends to understanding how every facet of health is affected when sleep is disrupted. The book’s contents are fundamental to all aspects of medicine, its recommendations are practical and relevant, and I recommend it to all doctors.

Book details

Publisher, Place, Year: Penguin Books, London, 2017
Number of chapters: 16
Number of pages: 360
ISBN: 978-0-141-98376-9
Star rating: 5
Price: £8.55

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