Why We Sleep by Matthew Walker: Summary & Notes

Front cover of Why We Sleep: Unlocking The Power Of Sleep And Dreams by Matthew Walker.

In short

In Why We Sleep Matthew Walker makes a very clear case for the importance of sleep. Even though many people probably understand that we need to get a good night’s rest, there is at the same time a glorification of not sleeping; of working long hours and sacrificing sleep throughout our lives. And even though this might not sound serious, sleep deprivation has significant consequences for our physical and mental health.

(And no, it’s unlikely that you’re one of those people who thrives on fewer than 8 hours of sleep since that’s an extremely rare trait. People underestimate the hours of sleep they really need.)

To be fair Why We Sleep did receive a lot of criticism since it’s release (for an overview see here). So maybe don’t take all of the messages in the book at face value. That being said, I don’t think anyone would argue that optimizing for better sleep is unhealthy, and so this book can still be useful in order to prioritize sleep in your life. You spend one-third of your existence doing it, after all.

Book Summary & Notes

All text that is quoted & italicized is taken directly from the book.


What is sleep?

Most people have a tendency to avoid sleep – it’s usually the first thing that gets sacrificed if you’re running out of time. And, considering the fact that we spend 1/3rd of our lives asleep, it does seem like a colossal waste of time. But the right question to ask is not so much “what is sleep good for?”, but rather: “what isn’t it good for?” Walker states that there are no known biological functions that do not benefit from a good night’s rest.

Biologically speaking there are two main factors that determine whether you want to sleep or stay awake:

  1. Circadian rhythm and our internal clock. All living creates have some sort of sleep-wake cycle, and for humans that cycle is about 24 hours long. The internal clock cycles between day and night rhythms, which explain why we feel alert during the day, and sleepy during the evening.
  2. Sleep pressure. The second factor relates to how long we’ve been awake for. During our waking hours a chemical starts to build up that essentially determines how sleepy we feel. The longer we’re awake, the higher the concentration of this chemical, and the more sleepy we will feel. (Caffeine battles the build up of this chemical, which is why you temporarily feel more awake and alert after drinking coffee.)

Not only do these two factors determine if we want to sleep or not, but they also determine how alert we are during the day, and how well we will be able to sleep.

There are some variations in these cycles; some people are night owls (about ~30% of the population), others are morning larks (about ~40% of the population), and the rest are somewhere in between.

Circadian rhythm and travel

As a consequence of our circadian rhythms we experience jet lag when we move time zones. I.e., we feel sleepy during the day, and awake at night – at least until your body adapts. This adaptation is slow: it takes one day for every hour of time difference.

Interestingly, it’s actually harder to readjust if you’re flying eastwards. There are two reasons for this:

  1. Flying eastwards means falling asleep earlier than normally. Staying up later is usually easier for people.
  2. Our circadian rhythm is actually slight longer than 24 hours (by about 15 minutes), so flying westwards is a bit easier to accommodate since there is some “slack” in our internal clock.

When do you get enough sleep?

Walker offers two simple questions that can roughly determine if you’re getting enough sleep:

  1. After waking up in the morning, do you think you could go back to sleep again in a few hours’ time?
  2. Can you function normally without any caffeine until noon?

If you answer the first question with a ‘yes,’ and the second with a ‘no’ then you are most likely not getting enough sleep (or not enough high-quality sleep). You could also do an experiment and don’t set any alarm clock: would you sleep way past your normal wake-up time? If so, then you probably don’t get enough sleep.

Two types of sleep

When we sleep we cycle through two different types of sleep: REM (rapid eye movement) sleep, and NREM (non-REM) sleep. In the first we exhibit brain activity that is almost the same as when we are awake, and this is when we dream. The second is when brain activity is low, when we are calm and relaxed.

The sleep cycle is comprised of 90 minutes, but the ratio between REM and NREM sleep actually changes throughout the night. The earlier part of the night is heavy in NREM sleep, while later parts of the night have more REM time.

Each type has a different purpose, but briefly put NREM functions “to do the work of weeding out and removing unnecessary neural connections. In contrast, the dreaming stage of REM sleep, which prevails later in the night, plays a role in strengthening those connections.” In a way you could say that NREM clears the ‘memory storage’, while REM builds it.

Another way of looking at is to “think of the wake state principally as reception (experiencing and constantly learning the world around you), NREM sleep as reflection (storing and strengthening those raw ingredients of new facts and skills), and REM sleep as integration (interconnecting these raw ingredients with each other, with all past experiences, and, in doing so, building an ever more accurate model of how the world works, including innovative insights and problem-solving abilities).

The nature of the sleep cycle also shows why it’s so bad to skip a few hours of sleep. If you have a short night, you will still get a large part of the NREM sleep, but you will miss out on a large part of the REM sleep. Walker gives the example of sleeping 6 hours instead of 8 hours – how much sleep will you lose? Mathematically 25%, but since the REM sleep is concentrated in later parts of the night you might actually lose 60 to 90% of all REM sleep.

How sleep changes when we age

Sleep patterns (obviously) change as we age. Adults mostly stick to monophasic sleep – meaning sleeping for one long period of time. In contrast, young kids sleep polyphasically: short periods of sleep throughout the day and night. The older the child gets however, the more stable (and longer) their sleep pattern becomes.

Young adolescents also go through an interesting period where their wake-sleep pattern shifts to waking up later and going to bed later. This is due to a change in their circadian rhythm: the period of wakefulness and alertness progressively becomes later and later. For parents this is often difficult to understand, but if you ask a teenager to go to bed to at 10 PM it’s actually similar to going to sleep at 7 or 8 PM for yourself. Likewise, waking up at 7 AM for a teenager is like waking up at 4 or 5 AM for an adult.

It’s a myth that older people need less sleep – they still need the same amount but it becomes more difficult for them to get it. The sleep quality and sleep efficiency is reduced, which for a large part is due to a reduction in deep sleep. Another reason is fragmentation – you wake up more often during the night. There’s also a change in older people’s circadian rhythm; it starts earlier and earlier (in contrast to teenagers).

Walker also makes a claim that many age-related physical and mental illnesses are related to sleep impairment. So achieving a full night of proper of sleep becomes more important as we age.

Why do we sleep?

There are many good reasons to sleep. One of the most clearly researched facts relates to memory aid: both sleeping before learning (to prepare the brain to make new memories), and after learning (to reinforce those memories) are highly beneficial. But it’s not just about remembering things, sleep also helps to ‘forget’ temporary / useless information, as well as to strengthen skills memories (‘muscle memory’).

Walker also gives the example of Usain Bolt who often takes a nap before the race – not only does this restore energy and remove fatigue, it also helps with his motor skills memories.

Sleep is also beneficial for creativity: while you are asleep your brain builds new connections between different pieces of information.

The dangers of not sleeping (enough)

It might be better to ask about the downsides of not sleeping vs. the benefits of sleeping due to the number of positive influences. Walker states that lack of sleep is linked to neurological and psychiatric conditions, such as depression, anxiety, Alzheimer’s disease, but also to all types of physiological systems and diseases, such as diabetes, cancer, obesity, and infertility.

One of the first things we experience with a lack of sleep is reduced concentration. This has major consequences, especially in the form of drowsy driving – in the US someone is killed every hour due fatigue-related accidents. In fact, being sleep-deprived for just a single night results in the same impairment as being drunk.

“Based on epidemiological studies of average sleep time, millions of individuals unwittingly spend years of their life in a sub-optimal state of psychological and physiological functioning, never maximizing their potential of mind or body due to their blind persistence in sleeping too little. Sixty years of scientific research prevent me from accepting anyone who tells me that he or she can “get by on just four or five hours of sleep a night just fine.”

There is also a link between sleep and psychiatric conditions. When it comes to depression, anxiety, PTSD, schizophrenia or bipolar disorder, the patients do not sleep normally.

“There are more than twenty large-scale epidemiological studies that have tracked millions of people over many decades, all of which report the same clear relationship: the shorter your sleep, the shorter your life. The leading causes of disease and death in developed nations — diseases that are crippling health-care systems, such as heart disease, obesity, dementia, diabetes, and cancer — all have recognized causal links to a lack of sleep.”

Not sleeping enough (which is the case for many adults) increases appetite, lowers impulse control, and thus increases food intake, while at the same time lowering feelings of satisfactions after eating and preventing effective weight loss when on a diet.

Poor sleep is also linked to cancer development, and with established cancer it can lead to faster growth of cancer cells. Walker compares it to pouring fuel on a fire, and the World Health Organization has apparently stated that nighttime work is a probable carcinogen.

Why do we dream?

While in REM sleep, and while dreaming, the brain builds new connections and consolidates memories. But dreams also seem to service two other purposes:

  1. It helps with emotional and mental health. In dreams we can revisit emotional concerns and re-experience traumas or problems without feeling frightened or emotionally disturbed. So it’s a sort-of overnight therapy.
  2. It helps with problem solving and creativity. While in REM sleep the brain will blend together the massive amount of information that we’ve acquired and connected distant pieces of information. This can result in novel and creative solutions. (This is also why a lot of creative people credit their sleep and dreams for coming up with something novel. One of the best examples of this is Mendeleev’s periodic table which he first saw in a dream.)

How to improve sleep (in modern society)

That sleep has a lot of benefits, and that not sleeping (enough) has a lot of downsides is clear. But how do we improve our sleep? Walker offers a number of suggestions:

  • Reduce caffeine intake (since it stops the build up of sleep pressure).
  • Reduce alcohol intake – it might make you fall asleep faster, but is detrimental to the quality of REM sleep.
  • Remove screens from the bedroom. Most devices emit ‘blue light’ that significantly reduce the production of melatonin, which is a chemical needed to feel sleepy.
  • Create a cool bedroom. The ideal temperature to sleep in is around 18 ℃ / 65 degrees Fahrenheit.
  • Establish a regular sleeping schedule. Go to bed and wake up at the same time every day (even on weekends).
  • Go to bed when you’re sleepy and avoid napping in the early/mid evening (which will disturb your sleep schedule).
  • Try to avoid daytime napping if you can’t sleep at night. Daytime naps have benefits, but only if you’re able to sleep normally at night.
  • Try not to worry about things before going to bed. Anxiety doesn’t combine very well with sleeping peacefully.
  • Remove alarms and clocks from the bedroom, so you can’t check what time it is (to stop anxious clock-watching).

For companies and organizations there is also a cultural change that is needed. Sleeplessness as a success factor, and the glorification of making long days with late meetings, do not lead to more productive, happier employees. In fact, it’s the opposite: sleep deprivation lowers productivity, happiness, and creativity, while increasing laziness and unethical behavior.

In short you could say that everyone seems to underestimate the importance of sleep. But a lack of sleep can cause various mental and physical problems. If there’s one takeaway from the book its this: make sure you develop healthy sleep habits and rest at least 8 hours a night. Obvious perhaps, but not easy to do.


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