Why We Sleep

Summary

Why We Sleep is an exploration of why and how humans need to sleep and an ode to all the beneficial aspects that sleeping has on our health. Healthy sleep (enough and good quality) is probably the single biggest contributor to a good life that we know of. The second most important would be exercise. This book goes into the weeds and shows why sleep is so goddamn healthy and necessary for us (and many other animals) to work.

The book provides evidence-based insights into the effects of sleep on physical and mental health. It also offers practical advice on improving sleep quality and explaining what sleep actually is and how it works. As such it belongs into a category of content that helps you live a better, more healthy life, right along with books such as Outlive by Peter Attia or podcasts such as that of Andrew Huberman.

Key Ideas

Sleep is comprised out of different phases. The two big categories are REM and NREM sleep. These phases are still studied in detail but it is clear that we need both phases of sleep for different reasons.

NREM sleep is associated with deep, restorative rest and involves slow-wave sleep, which is essential for physical restoration, immune health, and memory consolidation. We think that it helps transfer memories into long term storage, which is why some Alzheimer's like symptoms in old people can be cured when they are simply sleeping longer.

REM sleep on the other hand is linked to dreaming, emotional processing, and creative problem-solving. REM sleep allows the brain to process and integrate emotions and memories, which is critical for mental and emotional health. It also permutes experiences of our normal day to day life in creative ways, creating ideas and letting the subconscious work on problems we encounter. Often insights can come from REM sleep and sleeping a night over important decisions often helps making better ones.

These phases are not equally spread out through the night, so if we miss sleep in the morning we miss most of our REM sleep, whereas if we miss sleep in the early part of the night we miss out most of our NREM sleep. This further aggravates the damage done by not sleeping enough.

We–like many other animals–have a circadian rhythm. Interestingly, this rhythm is independent of the day and night cycle (even people in a room with no sunlight or blind people have it) and it is slightly longer than 24 hours. However, our circadian rhythm is still linked to daylight, because our eyes are sensitive to light and reset the circadian clock when we see bright light, particularly after waking up. This is sensitive to all wavelengths but predominantly towards blue wavelengths, which is why viewing blue light led screens in the evening is so toxic to our sleep. It suggests to our brains that it is still bright day, delaying the onset of melatonin, a neurotransmitter that makes us feel sleepy.

The process of learning is significantly impaired by insufficient sleep, as the brain becomes less capable of absorbing new information and making connections. Sleep before and after learning is necessary for optimal retention and understanding.

Everybody needs between 7-9 hours of sleep, there is no exception to this. People who sleep less, deal one way or another with the negative consequences of their lack of sleep. If they were to sleep longer, they would perform much better across a variety of tasks and have overall better health.

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.

Matthew Walker argues that society's sleep loss epidemic has wide-reaching consequences for public health, workplace productivity, and economic performance. He advocates for cultural and institutional changes, such as adjusting school start times and creating work environments that respect the importance of sleep for employees.

It is disquieting to learn that vehicular accidents caused by drowsy driving exceed those caused by alcohol and drugs combined.

Instead our current culture prides itself on the idea of not needing much sleep. Many consider it a sort of status symbol or something to strive for. However nothing could be further from the truth. Sleeping, as this book argues, is one of the core things we can do to live a better, more healthy life and time spent asleep is time well spent. In line with this Walker also gives a recipe for how to sleep better (which other health influencers like Andrew Huberman or Bryan Johnson also confirm and mirror).

Recipe for Good Sleep Hygiene

Go to bed the same time of day and wake up around the same time of day too. Be consistent with sleep cycles. Time in bed is not the same as time sleeping, usually efficiency is around 90% i.e. if you need 8 hours of sleep you need around 9 hours in bed.

In the morning:

• view bright outside lights for at least 15 minutes
• do some cardio intensive exercise
• take a cold shower (or have another form of cold exposure)
• don't ingest caffeine for at least 2 hours after waking up
• eat a meal early in the day

In the evening:

• don't raise heart rate too much, no movies or video games, also no exercise 4-6ish hours prior to bed
• take warm showers before going to bed
• turn off all screens and bright lights (including smartphones) around 2 hours before going to sleep
• don't eat like 4-6 hours before going to bed
• make the room completely dark and cold (around 16°C is perfect)

Detailed Notes

Sleep is really really really important.

It governs our mood, health, performance, memory, immune system - basically our whole life. Almost everything that has a brain has something very similar to sleep. This mechanism seems to be universal and evolved multiple separate times proving how important it really is.

When not sleeping enough one effectively diminishes the life one lives. Even though it is a considerable commitment in time - the advantages are well worth it. Without it one can simply not form memories anymore and is incapable of doing much and finally dies.

How we become sleepy - there are two mechanisms within our brain which determine when we feel sleepy and when alert - one is the circadian rhythm and the other is sleep pressure mediated by melatonin. Circadian rhythm is an internal clock that is situated in the suprachiasmatic nucleus that is roughly 24 hours - i.e. one day long - in most people a bit longer. A few scientists discovered by living in a cave for weeks/months that this internal clock needs a little readjusting because it is longer than 24 hours on its own, more like 24,5 to 25 hours - but the light of day and absence of light at night adjust it to fit the day-night cycle decently. This mechanism exists independently of outside stimuli. It is a truly internal clock. However it gets information about light from the outside and uses that to adapt - the suprachiasmatic nucleus basically readjusts this time frame depending on the light exposure we get.

Usually it has a ~12 hour period of activity and higher heart rate etc. during the day phase and an equally long interval of lessened activity during the night. When the activity is low - we tend to feel sleepy. This can be put into a cycling so e wave into a diagram - when the wave rises we feel alert and awake, when it falls we feel drowsy and sleepy. This rise and fall of alertness is correlated with the concentration of melatonin in our blood. Circadian rhythm triggers the release of Melatonin in the evening around 6 so that it peaks around nighttime. Then it falls down again in the early morning until we wake up. When having strong light exposure in the evenings though - in other words - looking at our bright smartphone or TV screens - this release of melatonin is pushed backwards in time and hence we feel less sleepy and do not go to sleep as we should.

The other mechanism controlling sleepiness is sleep pressure mediated by adenosine. Adenosine builds up within the brain during waking time and decreases while we sleep. It much like the low activity phase of the circadian rhythm causes the feeling of drowsiness. Therefore we are more awake after we have just woken up - adenosine levels have dropped during the night and so we have very low ones in the morning and feel super alert and awake. Ideally the circadian phase also ramps up at that moment and increases to its peak slowly. Meanwhile adenosine builds up while we go through our day. At some point it might be rather high and we start to feel drowsy so we drink a cup of coffee which blocks the receptors for adenosine and therefore we stop feeling sleepy for the time the caffeine is in our brains. However adenosine increases even throughout being under the influence of coffee only that we do not feel its effects because the receptors are blocked by the caffeine.

At some point we digested the caffeine and suddenly can feel the adenosine sleep pressure again - which is now even higher than before - so we feel even more sleepy. So we drink more caffeine. Or we take a nap. Now while napping our brain processes some of the adenosine and so the levels of it after our nap are less and hence we feel more awake again. After around 8 hours of sleep all of the adenosine on a human brain is cleared away. On the other side however as the evening approaches and we become drowsy because our activity circadian rhythms becomes low - we do not feel as sleepy because adenosine is lower than had we stayed awake throughout the whole day. Hence we do not feel as tired and might have trouble going to bed early and falling asleep.

Sleeping itself is a more complex phenomenon as well - it is not the same but cut into very distinct phases of light sleep deep sleep and REM sleep.

The activity of the brain is measured in electrical brain waves and a awake brain has highly irregular highly unpredictable and essentially random "waves" with changing amplitudes and frequencies. This is tied to insights of consciousness. Deep sleep however has highly predictable and stable waves. Deep sleep - also called NREM sleep is the whole brain pulsing and firing together rhythmically over and over again. NREM is sleep without dreams - but our brains are still connected to sensory information from the body.

The other sleep phase is REM sleep - the activity of the brain looks just like that of a waking person's brain - however the body is distinctly cut off from the rest of the brain. It does not move and sensory information is not being processed - the only muscles twitching during REM sleep are those that move the eyes and those which are automatic und unconscious and keep us alive - heart, lungs, intestines etc. Bot of these phases repeat each other night after night in 90 min intervals - with a higher amount of NREM in the first half of the night and a higher amount of REM during the second night. The interval in which they oscillate stays at 90 minutes however the amount occupied by each stage changes during the night.

So when we miss out on sleep we do not miss out on a couple of hours but we miss out on either most of our REM, when we wake up to early, or on most our NREM when we go to bed to late.

Sleep is old–500 million years at least because we know that worms do sleep and they have been evolutionary almost unchanged since that long ago. Sleep need and why animals have to sleep is not really understood yet. Animals have to sleep wildly different amounts without any clear correlation to anything. Body size, metabolic rate, complexity of the nervous system, or social environment–nothing matters. Some animals sleep long, others short. Giraffes sleep about 4 hours the brown bat 19 hours–that is how wildly different sleep needs are in different species. There is no real pattern to this... Fun fact: baboons sleep the same amount as guinea pigs - 9.4 hours.

Some animals have weird sleep abilities - whales and dolphins can sleep with half their brains and do not enter REM sleep. Seals can have REM sleep however only when they are out of the water - even though they show perfectly normal sleep otherwise while in the water. Birds can also sleep with half their brains and even organize within groups so that only the outside of a flock stays half awake and therefore the flock maximizes numbers of hours slept. Some birds during migration can even turn off their sleep need and power through sleep deprivation without having performance losses.

Humans rebound sleep - both NREM and REM - which means that when sleep deprived we will sleep for longer and have at first more NREM sleep and then in the subsequent nights catch up on REM as well having more of that. BOTH are essential though and hence we should get a good night's sleep every single night without exceptions.

Even though we rebound sleep this never gets the back all that we lost - therefore we lose some amount of sleep permanently when not sleeping enough and that is bad. Also our culture does not allow for healthy sleep patterns anymore. It seems that we have to little sleep overall - but equally worse we are active and sleeping during the wrong times - against our natural genetic tendencies. We should be sleeping biphasic - 6-8 hours at night from dusk to dawn and then again during the mid afternoon for another 2-3 hours. That would be optimal and what our biorhythms suggest.

Compared to other primates we have a lot more REM sleep compared to total amount of sleep. This is because REM sleep is almost impossible when sleeping in trees. This extra dense amount of REM rich sleep is causally linked to higher emotional and social intelligence and therefore shaped our evolutionary advantage significantly. It also fuels creativity because during REM distant memories and ideas get connected to form novel insights. The type of sleep we have is therefore one of the things that makes us human.

Autists have less and irregular REM sleep. Also when babies develop they have tons of REM sleep which is necessary for building up the scaffolding of a human brain. Analogy is that of an internet provider building the infrastructure for a whole new town. Building connections from everywhere to everywhere.

The amount of neural growth and the amount of REM sleep are unmatched and not found in any other phase of a human life. Furthermore alcohol disrupts this pattern - leading to several cognitive development problems in fetuses whose mothers are intoxicated with alcohol during pregnancy. The same is true for breastfeeding since alcohol is stored in the breastmilk - and disrupts the sleep of newborns after breastfeeding.

Babies do not have a fully formed suprachiasmatic nucleus and therefore a messed up 24 hour circadian rhythm. The nucleus only develops over time and that is why very young children have messy sleep/wake patterns. Also the amount of REM is decreased, while NREM increased. The high amount of extra NREM is logical since NREM culls neuronal connections and reinforces needed ones.

It is basically like the internet provider reshaping it's set of fiber optic cable based on usage statistics. This reshaping starts at the back of the brain and ends in the prefrontal cortex. This means the last part that gets updated properly and matures is the part of the brain that is necessary for producing rational thoughts. Which is part of the reason why teenagers behave the way they do. Loosing NREM or having poor NREM sleep is associated with all matter of cognitive problems including schezophrenia. Ingesting caffeine disrupts NREM sleep and degrades it's strength - both regularity and amplitude of the waves - significantly. Therefore caffeine is something that young people should not have because it will diminish the way their brain is shaped and developed - especially during puberty.

Furthermore teenagers circadian rhythm shifts to a later time which means they have peak wakefulness later during the evening and not only want but actually biologically have to go to sleep and wake up later than their parents. For children before puberty it is the opposite. Their circadian rhythms are shifted in the opposite direction. They then shift back once they are fully developed adults. Probably this is an evolutionary advantageous mechanism to segregate children from their parents - so that they go their own ways - within a group of other teenagers.

In old people sleep quality diminishes. They are awake more often through the night and worse - their circadian rhythms shift to an earlier time as well. Therefore melatonin is high and they want to sleep early in the evening but they doze off during that time and lose the sleep pressure of adenosine - and then when they go to bed later, can not fall asleep because they lack that pressure, so they fall to sleep way later than they should have and wake up very early in the morning, because their circadian rhythm kicks in and they become alert and awake, having not nearly enough sleep.

Also they wake up during the night because they have to go to the toilet more often - even further disrupting sleep. Overall they lose up to 90% of the NREM sleep a teenager gets per night - which is related to many signs of being old - like being more forgetful and less mentally acute.

This loss of NREM is because the brain part that generates the deep waves of NREM sleep gets damaged with old age - so old people literally lose the abitility to generate their own sleep. Numerous health and cognitive issues are linked to this fact and restoring this ability is a topic of ongoing research. People have been implanted with electrodes producing the pulse for NREM sleep and have shown reduction in symptoms of dementia and other age related cognitive diseases, only because they could sleep longer again. Old people need sleep and the author says we should give it back to them.

Sleep improves memory - it not just deletes memories that are unnecessary but also enhances the memories that are necessary. The NREM phase is the phase of sleep responsible for this and measuring the electrical sleep spindles activity within the brain is a good predictor of how well people can remember things they have learned on the day before they went to sleep.

Even better - when artificially enhancing those spindles - by outside electrical stimulation with the right frequency - or even by just having a sound play at the right frequency during sleep - one can enhance the memory quality of test subjects. Basically by improving sleep quality people can hack their memory. Even crazier, sleep can decide which memories are important and which are not - while asleep we interact with cues that determine what to remember gathered throughout the day. That means that memories that we deem important throughout the day get a different treatment during sleep than our other memories. During sleep memories get stored from the working memory within the hippocampus to another brain area called the neo cortex.

This can be compared to data from a USB disk being stored onto the main hard drive. Which of these memories gets stored is selected by importance and governed by multiple factors - e.g. number of repetitions - importance, significance etc. The actual process is not well understood yet and nobody actually knows how the information is transferred. We know however that persons who do not get to dump memories from their hippocampus daily working memory into this sort of long term memory during their sleep will start to "overwrite" the memories from the day before with memories from the day after. Hence they lose the ability to remember anything at all for a longer time.

Considering all this - sleep is very important for learning and repetition interspaced with sleep during the nights and a nap during the day seem to be the most promising approach for learning new things and concepts. Even things such as complex motor skills-learned within the cerebellum–profit from this sleep memory effect and there is a clear link between sleeping and improvements of motor skills that have been practiced before going to sleep.

Skills are retained better after sleeping than before sleeping even though more time has passed since then. And it is not only because of time passing that the neurons have time to grow because if persons learn the same thing and are tested for their skill with a similar time interval just without sleep–performance is still worse than compared to the group which sleeps. So, sleep is very important for the proper functioning of memory and therefore plays a critical role in learning.

The same is true for motor neurons learning and therefore the mastery of movement tasks. Those get revamped during the last 2 phases of early morning NREM and are affected by those phases intensity in sleep spindles. Implications are huge - give those last 2 hours to athletes and their rate of injury, recover, scores etc. their overall performance - just increases like mad. "If you don't snooze you lose". People who are really good at what they are doing, focus on getting solid sleep.

Sleep Deprivation is bad for the brain. Even when having only small doses of it the effects stack up over time and become as severe as if one where not sleeping at all - that means 10 days with only 6 hours instead of 8 yields the same cognitive performance as somebody who has not slept at all in 48 hours. Also, any amount of sleep deprivation leads to microsleep, where our brains basically shut down for the fraction of a second during the day - this causes a lot of accidents each year. Sleep deprivation is a real and scary issue.

Sleep recovery is also way slower than one would expect - it takes more than 3 nights to regain the same cognitive performance that one had before being sleep deprived on only 6 hours of sleep every night for a week. Hence when sleeping bad every week - this still accumulates in a downward spiral with so far no measurable bottom. And people are not aware of this. Even though cognitive performance is completely lost, people will be confident in their own competence immensely.

It is the same as a drunk person waving their car keys saying - "Yo I got this. I am fine to drive". Behavior like this is absolutely nuts and wrong on so many levels. Driving while sleepy belongs into the same category because the effects of sleep deprivation can be compared to alcohol. Sleep deprivation starts after being awake for 16 hours or more. That means that somebody who has been awake the whole day and spent the evening at a friend's place is as cognitively impaired as if he had a few drinks and should really not drive home. Matthew Walker begs people to stop driving when they are sleep deprived since it is costing the lives of many people every year.

Naps placed at the right time of day can delay the onset of the negative consequences of sleep deprivation but not for long. As to date there is no drug or behavioral hack whatsoever that can fight against the need to sleep efficiently. There is nothing to restore cognitive function indefinitely without sleep.

Sleep deprivation disrupts the interplay between prefrontal cortex - amygdala and striatum. This means that we get crazy mood swings when sleep deprived because our emotional centers are not kept in check by the thinking part of our brain. Which means that our reactions to both positive and negative events are much stronger emotionally than they would be had we had a full night of sleep. Those changes can be even linked to psychological conditions like depression and suicide rates, as well as other clinical psychological issues.

Some people with bipolar disorder have their manic/depressive phases triggered by sleep deprivation. Also there might be a two way link between clinical disorders and sleep deprivation - that is that probably both are causing and reinforcing each other to a certain degree.

Alzheimer has a strong link to sleep deprivation as well. When sleeping the brains glia cells shrink down and the production of cerebrospinal fluid increases - flushing out metabolites that gathered there during the day. These metabolites are exactly the same that make up the plaques which cause Alzheimer's disease. Namely beta-amyloids.

The author suggests that in the future we can prevent Alzheimer's disease or delay it's onset to a later age, by improving the sleeping habits and routines of older people. This would be similar like giving statins to somebody who is in the risk group for developing heart disease. Another crazy aspect of Alzheimer's disease is that the plaques tend to accumulate first in the prefrontal cortex - at just the place that would normally generate the healthy and necessary deep sleep NREM patterns. It would not be surprising if this primary destruction of the necessary sleep circuits in the brain - fuels the growth of Alzheimer's and makes the onset of dementia more rapid.

All nighters are very bad. When pulling an all nighter people lose the ability to learn new things during the next day. Hence especially students should never do them. The way we structure exams and learning periods leads to cramming and the pulling of all nighters and we should therefore get rid of this dangerous pattern. It is even worse because when pulling all nighters we can not regain this lost information and possibility if learning by "rebounding" sleep throughout the next days - during the time we are sleep deprived, we basically stop forming new memories in a way.

Sleep deprivation is damaging the body. It activates the sympathetic nervous system making for a strong stress response - releasing cortisol, which restricts arteries and making the heart beat faster and more often - overall this leads to hypertension - a dramatic increase in blood pressure, that over time damages the endothelium. Also HGH can not be released during night times sleep - reducing the repairing effects of sleep.

Overall the effect of even 1 hour less sleep is so drastic that it can be seen in the cardiac arrest cases each year when clocks are shifted one hour. Sleep loss is also causing obesity and diabetes. Cells in a sleep deprived body are less insulin sensitive so blood sugar levels vary more after a meal which puts stress on the beta cells in the pankreas which highly increases the chances of getting diabetes 2. Furthermore grhelin and leptin are regulated differently when sleep deprived - which leads to craving more food AND feeling less satiated when eating food - both of which lead to overeating of something like 300kcal each day.

Even worse, the bridge between control and reward center gets worse which means that impulsive cravings are more likely which leads to a consumption of more processed sugary food and calorie bombs. Also the microbiome changes in response to the cortisol levels in the blood - it gets worse when sleep deprived.

It can be clearly corelated that the number of hours slept went down over the last 60 years while the number of obese people went up and this seems to be a causally linked not only correlated. When trying to lose weight the body prefers to lose fat when having slept long but will loose muscle mass when being sleep deprived - so when wanting to diet - a healthy sleep is not just of importance but necessary for success.

Overall, Sleep is not a pillar of health... it is the foundation.