by Matthew Walker
Note: This Why We Sleep summary is part of an ongoing project to summarise The 35 Best Books on Health.
Why We Sleep Review
It’s not easy to turn a book of science into a compelling page-turner, but that is exactly what Matthew Walker achieves with his excellent Why We Sleep.
Is Walker sometimes guilty of inconsistent footnoting, misquotations and switching between fact and hypothesis without always alerting the reader?
Yes, he is. (For more, see this excellent article by Alexey Guzey.)
But does his exuberance come from the right place? Does he keep his enthusiasm mostly in check? And does he succeed in educating us about a topic that is poorly understood and causes untold needless suffering every year?
Yes, he does.
And for that, I can forgive Walker his errors.
Want a quick preview or recap of his main arguments?
Check out my Why We Sleep summary below…
P.s., I’ve heard a rumour that a second edition of Why We Sleep may be planned and will update this Why We Sleep summary accordingly if and when that happens.
Why We Sleep Summary
This Why We Sleep summary breaks down into 8 key questions:
- What Is Sleep?
- Why Do We Dream?
- What Makes Us Sleepy?
- Why Is Sleep So Important?
- How Does Sleep Change With Age?
- How Much Sleep Do I Need? (Quantity)
- How Can I Sleep Better? (Quality)
- But What If I’m STILL Struggling With Sleep?
Click on a link above to jump ahead or read on below…
What Is Sleep?
Sleep is a state externally characterised by:
- A stereotypical position (e.g., lying down);
- Lowered muscle tone;
- No overt communication or responsiveness;
- Being easily reversible; and
- Sticks to a reliable, timed pattern.
And internally (at least in humans) by:
- Loss of external awareness;
- Sensory blackout (directed by the thalamus, the brain’s sensory gate); and
- Time distortion (loss of conscious sense of time).
Despite this loss of awareness, sleep is NOT simply an absence of wakefulness.
Sleep is made up of three different, deliberate, metabolically-active states:
- Light NREM (stages 1 & 2) sleep;
- Deep NREM (stages 3 & 4) sleep; and
- REM sleep.
Light and deep NREM (non-rapid eye movement) sleep are characterised by:
- Eye movements: None;
- Muscle tension: Some (less than waking); and
- Brain waves: Deep, slow, and synchronised (rippling from front to back of brain).
NREM sleep has 4 stages, characterised by increasing difficulty of waking someone up:
- Stages 1 (lightest) & 2 are light NREM sleep; and
- Stages 3 & 4 (deepest) are deep NREM sleep.
Additional bursts of brain activity, called sleep spindles, also occur during NREM.
Sleep spindles are important in memory consolidation and play other roles, like blocking external noises (sensory gating).
They are richest in the last 2 hours of an 8-hour sleep and in parts of the brain that have worked hardest with learning during the preceding day.
REM (rapid eye movement) sleep is characterised by:
- Eye movements: Rapid, back and forth, left to right movement;
- Muscle tension: None (complete paralysis);
- Brain waves: Shallow, rapid and chaotic (similar to when awake)
REM sleep is the sleep state in which we dream.
Humans cycle between NREM and REM states every 90 minutes during sleep:
But the distribution of light NREM, deep REM and REM is not equal:
- The ratio of NREM to REM in adult humans is ~80/20; but
- Deep NREM is more prevalent at the start of the night; and
- Light NREM and REM are more prevalent at the end of the night.
We’ll discuss the functions and benefits of each sleep state later.
For now, though, let’s talk about…
Why Do We Dream?
We know a lot about HOW we dream.
We know that dreaming takes place during REM sleep.
We know that when we begin dreaming, 4 main areas of the brain become active:
- Visuospatial areas – Areas that deal with vision;
- Motor areas – Areas that deal with movement;
- The Hippocampus – Short-term and spatial memory areas; and
- Emotional centres – The amygdala and cingulate cortex.
And we know that at the same time, the prefrontal cortex (rational thought) gets deactivated.
But it’s the question of WHY we dream that has most interested philosophers and scientists for thousands of years.
Dreams are NOT psychological manifestations of repressed desires. Theories (including 19th Century Freudian and Jungian ones) of dream analysis are “nonscientific and hold no repeatable, reliable, or systematic power for decoding dreams.”
Dreams are also not purely autobiographical replays of events from the last 24 hours of our waking lives.
Instead, dreams play an important role in each of the main functions of REM sleep:
- As overnight therapy – Taking “the painful sting out of difficult, even traumatic, emotional episodes you have experienced during the day, offering emotional resolution when you awake the next morning”;
- For decoding emotional experiences – Acting like a tuning fork that helps us recalibrate the brain’s emotional compass (which helps us e.g., recognise important emotional cues in other people’s facial expressions); and
- As ideasthesia – Inspiring creativity and promoting problem-solving by integrating new memories with old ones, building connections between previously unconnected ideas and extracting abstract, general rules.
Note: There is hard (MRI-based) scientific evidence for the ability of some people to consciously control the contents of their dreams (lucid dreaming). There is, as yet, no evidence if or to what extent such interference in the natural process of dreaming is harmful or beneficial.
What Makes Us Sleepy?
Two main processes regulate when and how sleepy we feel:
- Process C – Our circadian rhythm, or the wake drive.
- Process S – Homeostasis, or the sleep drive.
Let’s break them down…
1. Process C – Our circadian rhythm, or the wake drive.
The first process that regulates sleepiness is a biological clock; our circadian rhythm.
The length of our circadian rhythms varies between individuals but runs, on average, slightly longer than 24 hours.
To help them keep time, our circadian rhythms are constantly recalibrated by “zeitgebers” (German: “time givers”) like light, activity, temperature and food.
The SCN regulates many processes in the body, including body temperature:
And the release of the sleep-wake cycle regulating hormone Melatonin:
Our circadian rhythm can adjust over time (e.g., when recovering from jet lag) but that adjustment is limited to ~1 hour per day.
Note: Walker suggests that the timing of peak wakefulness as driven by our circadian rhythm varies genetically among individuals with:
- 40% of the population being morning chronotypes;
- 30% of the population being evening chronotypes; and
- 30% of the population somewhere in-between.
But he doesn’t state the size of the effect or give footnotes to back up this claim.
2. Process S – Homeostasis, or the sleep drive.
As well as the circadian rhythm, all cells in the body release a compound called adenosine (as a byproduct of cellular respiration).
While awake, adenosine levels build up faster than they can be broken down (like lactic acid in anaerobic exercise).
The elevated adenosine interacts with receptors that make us increasingly drowsy.
The effect peaks after 12 – 16 hours of wakefulness, creating a strong urge to sleep.
During sleep, adenosine production decreases and the body is able to bring adenosine levels back down to levels more conducive to wakefulness.
Process C and Process S are independent, but their effects support each other to help regulate sleep.
The Sleep Switch
There is a powerful sleep-wake switch in the brain, regulated by the chemical orexin.
When we are awake, the hypothalamus (which contains the SCN) releases orexin, which tells the brain stem we are awake.
The brainstem stimulates the thalamus (the brain area above the hypothalamus), which opens the sensory gate between the brain and the outside world.
When we fall asleep, the hypothalamus stops releasing orexin, which powers down the brain stem, which closes off the sensory gate.
It is the malfunctioning of this pathway that causes the sleep disorder, narcolepsy.
The body goes to a lot of effort to make sure we sleep, which begs the question…
Why Is Sleep So Important?
Getting enough sleep/not being sleep deprived:
Health & Vitality:
- Helps you live longer (protects DNA);
- Reduces cravings (e.g., food, alcohol, drugs);
- Protects you from colds and the flu (boosts immune system);
- Protects you from obesity and diabetes (improves metabolism);
- Protects you from cancer (boosts cancer-fighting immune cells);
- Lowers risk of hypertension, heart attack and stroke;
- Increases release of growth hormones;
- Significantly reduces the risk of athletic injury;
- Restores athletic strength and endurance;
- Decreases sensitivity to pain;
Note: For more statistics and details on the effects of sleep deprivation on physical health, see chapter 8 in the book.
Thoughts & Emotions:
- NREM: Protects you from Alzheimer’s1;
- Improves emotional resilience and stability2;
- Lowers risk of anxiety and depression3;
- Lowers risk of bipolar episodes3;
- Lowers risk of suicide3;
Note 1: Glial cells shrink ~60% during NREM allowing cerebrospinal fluid to flush out toxic debris like amyloid proteins, tau proteins and other molecules.
Note 2: Sleep deprivation hampers the prefrontal cortex (reasoning) from inhibiting responses in the amygdala (fear, anger) and striatum (impulsivity, reward), increasing the extremity of both negative and positive emotional responses.
Note 3: Though 1 night of sleep deprivation can improve depression in 30-40% of sufferers, the effect goes away after sleep and the 60-70% who don’t improve will actually get worse.
Growth & Learning:
- NREM: Restores the brain’s capacity to absorb facts (declarative memory);
- NREM: Selectively strengthens (consolidates) existing declarative memory;
- NREM: Repairs and recovers declarative memories;
- Stage 2 NREM: Refines and maintains non-declarative (e.g., motor) memory;
- REM: Promotes complex memory assimilation and integration;
- REM: Improves abstract problem solving and boosts creativity;
- May play a critical role in recovery after brain damage (e.g., from stroke);
Note: Walker dives more deeply into sleep and memory. For more on e.g., the role of sleep spindles in declarative memory transfer between the “short-term” hippocampus and the “long-term” neocortex or the role of sleep in non-declarative memory consolidation, see chapter 6 in the book.
Note: Pulling an all-nighter decreases the brain’s ability to learn new facts by as much as 40% and any facts learned are forgotten more quickly.
Family & Friends:
- Improves safety (e.g., while driving);
- REM: Improves emotional/allocentric thinking;
Note: Walker talks at length about the underreported import of drowsy driving. Short story: if you feel drowsy, get off the road, nap 20-30 minutes, wait 20-30 minutes, then, if you must, have a coffee and start driving again.
Just remember: When you’re tired, you’re bad at predicting how tired you are which is just one reason drowsy drivers are often more dangerous on the road than drunk drivers. For more, see chapter 7 in the book.
Love & Partnership:
- Makes others rate you as more physically attractive;
- improves libido (increases testosterone);
- Increases sperm count;
- Increases ovular fertility;
Productivity & Performance:
- Reduces drowsiness;
- Improves concentration;
Business & Career:
- Improves complex reasoning; and
- Improves rational decision making.
And these benefits ripple out across the whole of society.
So, sleep is critical.
But before we talk about how much you need and how to sleep well, let’s review…
How Does Sleep Change With Age?
Our sleep preferences and ability to sleep change dramatically throughout life.
- Pre week 23: Sleep-like state, much of which resembles REM.
- Post week 23: Brain centres wired up for NREM and REM. Foetus spends 6h in NREM, 6h in REM (but without paralysis, hence kicking) and 12h in an intermediary (neither clearly REM nor NREM) state.
- Final trimester: First glimmers of true wakefulness (2 – 3 hours per day).
- Last two weeks: REM up to 9h per day as synaptogenesis (brain development) starts to peak.
- Last week: REM up to 12h per day.
Note: Maternal drinking disrupts foetal REM. Heavy drinking has been linked to increases in neuropsychiatric illness. But even 2 glasses of wine is enough to significantly disrupt foetal REM and breathing while alcohol is in the mother’s system.
- First 3-4 months: Highly polyphasic (lots of short naps) as SCN develops.
- At 6 months: An infant sleeps ~14 hours per day with a 50/50 NREM to REM ratio.
- 4 months – 1 year: Sleep gets less polyphasic as circadian rhythm strengthens.
- 1 year – 4 years: Sleep tends to natural biphasic (night + afternoon nap) pattern.
- At 5 years: A child sleeps for 11 hours with a 70/30 NREM to REM ratio.
- Late childhood: Sleep trained into modern monophasic (night only) sleep pattern.
- During childhood: Circadian rhythm tends to peak early in the day (sleep signals peak ~9 PM).
- Pre-puberty: NREM peaks as brain maturation (pruning) kicks in.
- During puberty: Circadian rhythm gets later (2-3 hours after a normal adult).
- Late teen/adulthood: People sleep for ~8 hours with an 80/20 NREM to REM ratio.
- Young/middle adulthood: Circadian rhythm gets earlier again (sleep signals vary by individual).
Midlife and old age:
Older adults do NOT need less sleep than younger adults.
And yet, as we get older:
- NREM declines – We get less deep sleep (-60-70% by mid-40s and -70-80% by mid-80s vs. young teenage years);
- Sleep gets fragmented – Caused by disease, medication, bladder weakness; and
- Sleep timing gets disrupted – Circadian rhythm gets weaker and shifts earlier.
Failing to adapt causes sleep efficiency (time asleep/time in bed) to decrease dramatically (~70-80% by mid-80s vs. 95% in teens).
Causing a decline in both quantity and quality of sleep.
Which may be a significant contributor to age-related health issues.
In fact, Walker goes as far as asserting that poor sleep may be one of the most under-appreciated factors in cognitive and medical ill-health in the elderly.
How Much Sleep Do I Need? (Quantity)
To get the full benefits of sleep, you need enough of it to:
- Give each sleep state time and space to work; and
- Bring your adenosine levels back down to a workable level.
How much sleep is that?
You’d need a lab to get an accurate answer since sleep requirements vary between and within individuals based on genetics, activity levels, sleep efficiency, age etc…
But as a rough guideline:
- Walker gives himself an 8-hour sleep opportunity (time in bed) every night;
- And the CDC suggests spending at least 7 – 9 hours in bed every night.
Note: A very rare group of individuals (you are more likely to be struck by lightning) can get away with less than 5 or 6 hours of sleep. Though the exact mechanism is not yet understood, it seems to be linked to a sub-variant of gene BHLHE41.
As an optional bonus (not a substitute), you might also take a 30-60 minute nap in the early afternoon.
Note: There is evidence to suggest humans are naturally biphasic (night plus early afternoon nap, not two night time sleep phases – this was a 17/18th Century fad). And daytime naps contain sufficient sleep spindles to offer significant memory improvement as well as boosting perceived energy and reducing muscle fatigue.
How do you know if you need more sleep than you’re getting?
The human brain is very bad at estimating how sleep deprived it is.
So here are two questions you can ask to check if you’re sleep deprived:
- Could I fall back to sleep at 10 or 11 AM? If so, you probably need more sleep.
- Can I function optimally without caffeine before noon? If not, you’re probably medicating sleep deprivation and need more sleep.
Still not sure if you’re sleep deprived?
Here are a few more bonus questions to double-check your suspicions:
- If you didn’t set an alarm clock, would you sleep past that time?
- Do you find yourself reading and re-reading the same sentences?
- Do you often forget what colour the last few lights were when driving?
If you answered yes to any of these questions then you may want to stop for a nap and/or review your ongoing sleep habits.
Note: It’s important to make sure you don’t cut sleep short as doing so may cause you to miss out on the richest period of late-stage light NREM sleep spindle activity that is essential to memory consolidation.
If you need an alarm to wake up, consider going to bed earlier!
Is it true that too much sleep leads to an early death?
Most things that are good for you – eating food, drinking water, breathing oxygen, sleeping – will kill you if you take them to ridiculous extremes.
But Walker suggests that studies which show that as little as 9-hours of sleep are linked to increased mortality may be misleading since it’s very unclear if it’s lots of sleep that causes the cited causes of death (esp. infection and immune-activating cancers) or whether it’s the conditions which cause the body to try and heal itself with more sleep.
One thing we do know, however, is that quantity is not the only important factor in getting enough sleep. It’s also essential to think about…
How Can I Sleep Better? (Quality)
When it comes to getting enough NREM and REM, quality of sleep is as important as quantity of sleep opportunity.
The body uses several internal and external signals (see zeitgebers in “What is sleep?” to help us transition into healthy, high-quality sleep.
When these signals are disrupted, the result is lower sleep efficiency and (often) sleep deprivation.
Some disruptions are natural (e.g., age-related physiological changes, nursing children).
Others come from or have been made worse by, our modern lifestyles.
Among them are 5 main offenders, including:
- Electric (especially LED) light – Disrupts SCN calibration of our circadian rhythm;
- Regularised temperature – As above, temperature (especially night-time cooling) is a powerful circadian zeitgeber;
- Caffeine – Blocks adenosine detection, masking fatigue and disrupting sleep;
- Alcohol – Acts as a sedative but disrupts REM and fragments sleep; and
- A legacy of punching time cards – Cuts sleep short, killing late-stage sleep.
To maximise sleep quality, Walker makes 12 suggestions:
- Stick to a sleep schedule – Go to bed and wake up at the same time every day.
- Exercise daily – At least 30 minutes but not within 2 – 3 hours of sleep.
- Avoid caffeine1 and nicotine – Stimulants disrupt your sleep.
- Avoid alcohol2 before bed – Alcohol impairs REM and fragments sleep.
- Avoid large, late night meals and drinks – They can disturb and disrupt sleep.
- Avoid medications that disrupt sleep – Check for alternatives.
- Avoid naps/dozing after 3 PM – They make it harder to fall asleep later.
- Relax before bed3 – Make time to wind down before sleep.
- Take a hot bath – The post-bath drop in body temperature can help you feel sleepy.
- Create a dark, cool, comfortable, gadget-free space – Make sleep as easy as possible and reduce exposure to (especially LED) light4.
- Get 30 mins of sunlight per day – Natural light regulates your circadian rhythm.
- Don’t lie in bed awake – If you can’ sleep after 20 minutes, don’t lie in bed panicking, do something calming (like reading a book) until you feel sleepy.
Note 1: It takes 30 minutes for caffeine to take effect but 5-7 hours to process 50% of the caffeine in our system. Genetics play a role (some people have a fast-acting enzyme). As does age (older people metabolise caffeine slower). And it’s worth noting that decaffeinated coffee still contains 15 – 30% of the caffeine in a regular cup of coffee.
Note 2: Even a small quantity of alcohol (~3 units) consumed 3 nights after learning was shown to severely disrupt REM and cause partial amnesia of up to 40% of newly learned facts vs. a control group.
Note 3: For more on this, you may enjoy this article on how to master your morning routine (Hint: It begins with a solid evening routine.)
Note 4: If you must use screens later in the evening, Walker suggests installing software/activating features that de-saturate blue LED light on all your devices.
But What If I’m STILL Struggling With Sleep?
If you’re still struggling to sleep, despite following the recommendations above, it could be that you’re suffering from a sleep disorder like:
- Insomnia – Inadequate ability to sleep despite sufficient opportunity;
- Sleep apnea – Breathing difficulties that disrupt sleep;
- Somnambulism – Physical activity during NREM stage sleep;
- Narcolepsy – Sudden daytime sleepiness, sleep paralysis and cataplexy; or even
- Fatal familial insomnia – A very rare and incurable genetic form of insomnia.
Note: I’ll cover insomnia below. For the other disorders, see chapter 12 in the book.
Insomnia is the most common sleep disorder, affecting 1 in 9 people (though twice as common in women as in men).
It is clinically characterised by:
- Dissatisfaction with sleep quantity or quality;
- Suffering significant distress or daytime impairment;
- Takes place at least 3 nights per week for at least 3 months; and
- Does not have any coexisting, causative mental/medical conditions.
Is most commonly triggered by emotional distress and anxiety that keeps the body in a sleep-inhibiting fight-or-flight mode.
And comes in two forms:
- Onset insomnia – Trouble getting to sleep; and
- Maintenance insomnia – Trouble staying asleep.
Of which you may suffer from one, the other or both.
If you suffer (or suspect you suffer) from any sleeping disorder (including insomnia)…
DO NOT resort straight to sleeping pills (even light use) since they:
- Sedate you without promoting natural sleep;
- Encourage over-caffeinating to combat next-day grogginess;
- Can worsen forgetfulness and impair motor skills;
- Can lead to rebound insomnia which triggers dependence; and
- Are strongly correlated with increased cancer and mortality rates.
Note: For more on this, see chapter 14 in the book.
Instead, seek sleep-specialised medical help ASAP and explore treatments like CBT-I (Cognitive behavioural therapy for insomnia) which have been shown to be more effective (though less profitable for drug companies) in combating sleep deprivation.
Your sleep is too important not to fix.
P.s., Chapters 15 and 16 of Why We Sleep contain Walker’s thoughts on why solving sleep deprivation is of critical (and under-recognised) importance for society as a whole. He also discusses how governments and organisations could take steps to help. Though interesting, they’re not immediately, individually practical so I’ve omitted them from this summary. To learn more, see the original book.
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Enjoyed this Why We Sleep summary? You might enjoy the rest of the books on this list of The 35 Best Books on Health.