Why We Sleep - Matthew Walker
The New Science of Sleep and Dreams
I. This Thing Called Sleep
1. To Sleep...
More than two thirds of all people in developed countries do not get the recommended 8 hours of sleep.
[bq] “The shorter your sleep, the shorter your life span.”
Until very recently had no idea why we actually sleep. From an evolutionary perspective, sleep initially seems like a terrible idea since we don’t do anything useful and are vulnerable.
But the fact that every species sleeps shows that there must be tremendous benefits far outweighing the downsides.
Now starting to find that sleep has many crucial roles in health, not just a single one.
[bq] “Sleep is the single most effective thing we can do to reset out brain and body health each day.”
2. Caffeine, Jet Lag, and Melatonin
Two factors that determine when we want to/can sleep: Signal from inner 24h clock, as well as build-up of “sleep pressure”.
Circadian rhythm extremely important for controlling many things in our body.
Famous experiment by Kleitman and Richardson showed that humans both have an internal rhythm, independent from external triggers like sunlight, and that that cycle is actually slightly longer than 24 hours. —> Circadian rhythm.
External signal of sunlight resets our own signal to exactly 24 hours.
Also have other “zeitgebers” that act as triggers/cues for correcting our internal clock. E.g. environmental temperature.
Not everyone’s rhythm is the same. About 40% morning people, 30% night owls, and 30% in between. Different peaks and troughs in daily cycle, mainly caused by genetics. Night owls have it tough, often unfairly labeled as lazy and forced to get up at non-ideal times.
Fixed work schedules, which are biased towards early start, favour morning people, forcing night owls into unnatural sleep-wake cycles. Leads to them being chronically sleep deprived and functioning sub-optimally.
Melatonin responsible for telling the body when it’s time to sleep, but doesn’t induce sleep itself.
Jet lag is the body’s internal clock being “stuck” in a different time zone. Resets by about one hour per day.
Sleep pressure is caused by build up of adenosine in brain. Caffeine binds to adenosine receptors and blocks its sleepiness inducing effect.
Caffeine crash: Even if caffeine blocks receptors, adenosine still keeps building up, and as the caffeine gets processed by the liver, the adenosine rushes back to the now free receptors.
Two forces behind sleep, circadian rhythm and adenosine/sleep pressure, are not linked. We need to make sure they are aligned.
Am I getting enough quality sleep? Could I fall asleep again at 11am? Do I need caffeine to function before noon? If “yes” to either of these, probably not getting enough sleep.
Often lack of refreshedness a result of adenosine remaining in body even after sleep.
3. Defining and Generating Sleep
Some clear signals of sleep:
Tend to adopt horizontal position
Skeletal muscles relax
No sign of communication or responsiveness
Easily reversible (opposed to e.g. coma)
Fairly reliable time pattern
Self-perception of whether asleep or not:
Apparent) loss of external awareness
Distortion of time in two ways:
Time void: Suddenly wake up at a later time
Feeling of stretched time in dreams
Proper assessment of sleep only possible based on electrical signals from brain, muscles and eye movement.
Two key phases: Rapid eye movement REM (dreaming) sleep and non-REM, which is further sub-divided in 4 “depth” stages.
Go through roughly 90 minute cycles of NREM and REM sleep.
The longer we sleep, the higher the ratio of REM sleep per cycle.
One possible explanation for shifting ratio: NREM responsible for trimming down neutrons and weeding out redundant memories, REM for enforcing new memories —> Efficient way to optimise brain storage capacity.
Danger: If going to bed too late or getting up too early, shift ratio towards either REM or NREM and induce imbalance in this finely tuned mechanism. Even worse than just sleeping less overall.
[NFW: Possible explanation for my findings on REM ratio increased on alcohol/late sleep nights.]
Waking brain waves: high frequency (30-40 Hz) and chaotic, no order.
Reason: Independent activity all over the brain that sums up to apparent chaos.
Deep “slow-wave” sleep: Only ~2-4Hz; predictable pattern of slow waves, often followed by short noise bursts, “sleep spindles”, thought to protect the brain from external noise. Signal mainly generated in frontal lobe and travels from there to back of brain as if emitted by a speaker. Huge number of neutrons get synchronised during this phase.
Slow waves travel further, allowing communication between areas in brain that’s not possible during fast wave times. Allows moving memories from local short term storage to more optimal long term storage locations.
Waking: Information recognition
Deep sleep: Information transfer/distillation
REM sleep almost same brain activity as waking, some areas even show higher activity.
Memories are replayed and integrated with one another.
REM and waking almost indistinguishable from brain activity. But muscle activity completely different. Only during REM, muscles completely relaxed, leaving us paralysed. Prevents us from acting out dreams.
4. Ape Beds, Dinosaurs, and Napping with Half a Brain
Surprisingly, all animal species studied so far exhibit something akin to sleep, even the simplest worms and molluscs.
Even unicellular organisms have active and passive phases aligned with light cycle that might have been precursor to sleep.
But also differences.
Duration: Elephants sleep only 4 hours, some bats up to 19 hours a day. Reasons still largely unknown.
Only birds and mammals, which evolved later, have proper REM sleep. Suggests that REM sleep evolved later; Exception: No REM sleep detected in aquatic mammals (e.g. dolphins), possibly because paralysis would make them drown
[bq] “Absence of evidence is not evidence of absence.”
After sleep deprivation, body tends to have higher ratio of NREM sleep on first night, but higher REM on following nights.
However, can never truly make up for lost sleep, no matter how much we sleep afterwards.
Aquatic mammals sleep with only half their brain at a time to maintain vital movement. Shows how essential sleep is. Same in birds. They keep watch for threats with one eye while the other half of brain sleeps. In flocks, some keep guard by “half-sleeping” while the centre can sleep fully. But only possibly for NREM. REM always occurs in both halves simultaneously.
Pressures that can temporarily prevent sleep also vary across species. E.g. birds with transoceanic migration don’t sleep during the trip.
The way we sleep is also no longer as it used to be. Monophasic sleep not natural. Afternoon dip in alertness used to be time for nap.
Also midnight used to be middle of sleep phase, now it’s often not even the beginning.
Modern society has divorced us from our ingrained sleep patterns.
Two long sleep phases per night is a myth. Was common in 17th/18th century, but a cultural trend, not something biologically neutral.
Natural sleep aligned to biological rhythm: Long sleep at night and shorter mid-afternoon nap.
Greece massively abandoned siesta culture around year 2000. Studies showed that mortality due to cardiovascular disease rose by up to 60%.
Humans sleep much less than other primates, but much higher rate of REM sleep. All other primates sleep in trees. Doesn’t allow for same sleep depth, and paralysis during REM sleep as in humans.
Homo erectus was first to use fire and allowed it to safely sleep on the ground. Shorter duration but increased intensity would further reduce risk.
This re-engineering of sleep might be what made homo sapiens so dominant and “smart”. Two key factors: high social complexity and high cognitive intelligence. REM sleep boosts both.
Processing emotions maybe most powerful gift of REM sleep that allowed us humans to build such complex societies.
Creativity similarly uniquely reliant on REM sleep and responsible for our “superiority”.
5. Changes in Sleep Across the Lifespan
Fetus in womb sleeps almost entire time, in REM like state. Paralysis not developed yet, so it moves and kicks.
During early life lots of REM sleep. Critical for constructing the brain, and not yet that much need for maintenance. Sleep disruption at this stage particularly bad long term consequences. Could be one cause of autism.
Alcohol consumption of mother disrupts foetus’ REM sleep and leads to neurological underdevelopment.
Alcohol concentration in breast milk similar to that in blood, and can also disrupt infants sleep/development.
Circadian rhythm takes time to develop, and newborns sleep in short, irregular phases through night and day. Takes at least until age 4 for circadian rhythm to properly develop.
In adolescence deep sleep intensity starts being strongest at back of brain and gradually moves forward towards frontal lobe. Brain development follows same pattern as a result.
Pruning behaviour of deep sleep particularly important for adolescents and lack of deep sleep can lead too many developmental issues, including schizophrenia.
Teenagers’ circadian rhythm actually shifted to later times, with ideal sleep time being later than adults. Early school starts have terrible consequences.
Asking a teenager to get up at 7 am and function well is like asking an adult to do the same at 4 am. But society not designed for this.
Older adults need just as much sleep as during midlife, but often struggle/fail to generate same amount of sleep.
Deep sleep starts to reduce in quality and quantity from early 30s.
In elderly, many of the common health issues are actually results of sleep issues.
Circadian rhythm also shifts to earlier times again. But early evening snooze many elderly enjoy has severe downside, releasing sleep pressure and messing with their nighttime sleep.
Poor memory and poor sleep in old age highly correlated.
II. Why Should You Sleep
6. Your Mother and Shakespeare Knew
If there was a drug with the same benefits as sleep it would be hailed as a miracle drug.
Sleep before learning refreshes our ability to make new memories by moving existing memories our of the hippocampus into the cortex. Sleep spindles seem responsible for this, and amount of sleep spindles and learning ability seem directly related. More sleep spindles —> higher restoration in learning capability.
Amount of deep NREM sleep direct predictor of how much fact based knowledge someone will recall after a night’s sleep.
Long range communication allowed by deep sleep moves memories from hippocampus to long term neocortex.
Can even resurfacer memories that were thought to be forgotten. Sleep therapy might help memory reactivation, e.g. in case of dementia.
Can use transcranial direct current brain stimulation (tDCS) to boost slow waves in deep sleep and increase number of sleep spindles. Auditory signals, if timed right, can do the same.
Sleep also important for selectively remembering only important memories, or even forgetting “bad” ones such as traumatic events or addictive behaviours/patterns.
Also crucial for motor skill memory. Musicians often experience this. Can’t stick a particularly tricky part no matter how hard they try, but suddenly after a night’s sleep it flows easily.
Sleep makes motor skills automatic, and specifically reinforces difficult parts.
Last 2 hours of and 8 hour sleep particularly good for this.
Crucial to top athlete performance.
[bq] “If you don’t snooze, you loose.”
Also found correlation between shorter sleep and higher injury risk in athletes.
Crazy how much money sports teams spend on optimising nutrition and healthcare for their expensive players but then completely ignore sleep.
7. Too Extreme for the Guiness Book of World Records
Lack/loss of concentration one of the first consequences of too little sleep. Drowsy drivers responsible for huge number of traffic deaths.
Microsleeps, becoming consciously unresponsive start to occur with only a bit of sleep debt.
Sleep debt additive! Sleeping only 6h per night for around ten days shown to be just as bad as a single night without any sleep. And just keeps accumulating from there.
Self-perception of lack of sleep extremely bad. Tend to overestimate how well we can still perform. Also quick resetting of baseline. Many spend decades of their life in suboptimal physical/mental state without even knowing.
From all the science: People who claim to only need 4 or 5 hours to function optimally are a myth!
Staying up for 19h as bad for cognitive ability as being legally drunk! Equivalent to waking at 7am and going to bed at 2am. Cause for many car accidents.
After just 16 hours awake, brain starts to weaken in many aspects.
Naps most effective before sleepiness sets in. Prevention rather than cure. A tiny fraction of people (<1%) has a genetic mutation that allows them to keep their cognitive ability with just 6h of sleep per night.
With little sleep, amygdala takes over, leading to inappropriate emotional responses, anger, reactivity, and fight-or-flight response. Rational control from prefrontal cortex dims down.
But not just negative emotions amplified, positive ones as well, leading to risk taking and addictive behavioural reward cravings.
Sleep issues also closely linked to many types of mental illness, and sleep therapy often effective at controlling the symptoms.
[q] “The best bridge between despair and hope is a good night’s sleep.”
- E. Joseph Cossman
Sleep needs to happen the same day as learning for a memory consolidation effect. Can not “catch up” on sleep in this way.
Lack of sleep key lifestyle indicator for developing Alzheimer’s disease. Alzheimer’s often comes with sleep disruption, and the two form a vicious cycle. Cause of Alzheimers is amyloid build-up in brain. Particularly in frontal love, which is also responsible for generating the slow waves of deep NREM sleep.
NREM sleep also responsible for cleaning out toxic byproducts of wakefulness, including amyloid, from brain. During deep sleep brain cells shrink up to 60% allowing cerebrospinal fluid to flush out toxins.
8. Cancer, Heart Attacks, and a Shorter Life
Lack of sleep, especially from mid life onwards, terrible for cardiovascular health and blood pressure.
Sympathetic nervous system responsible for putting body in overdrive in extreme stress or danger situation. Should usually only be active for minutes or hours at a time, but if sleep deprived may be chronically active, causing lots of damage. Dramatically increases cortisol levels.
Even just an hour less of sleep, as seen yearly with shift of daylight saving time, leads to significantly higher risk of stroke or heart attack the following day.
Lack of sleep makes cells insulin resistant, causing higher blood glucose levels and ultimately diabetes.
By disrupting leptin and ghrelin levels, lack of sleep also makes us crave more food.
Sleep helps keep gut microbiome healthy.
Lack of sleep reduces both testosterone and sperm count.
Even our attractiveness as perceived by others measurably decreases after a single night of short sleep.
Sleep boosts immune system, makes infections less likely, and even improves our response to vaccines.
Cancer fighting also occurs during sleep, and even just brief sleep deprivation can severely disrupt this.
Study found that consistently sleeping 6h or less leads to 40% increased cancer risk.
Reason two-fold: Reduced killer cells lead to less cancer fighting, and ramped up sympathetic nervous system leads to chronic inflammation which feeds cancer cells.
Sleep also stabilizes genes, and lack of sleep alters gene expressions.
Sleep also protects telomeres. Telomere length essentially predictor of “perceived age”.
III. How and Why We Dream
9. Routinely Psychotic
At first glance, dreaming seems like a psychotic episode: Hallucinations, delusions, disorientation,…
(Almost) all dreaming happens in REM phase.
Brain areas that are particularly active in REM sleep: visuospatial regions, motor cortex, hippocampus, emotional centers of brain.
But also areas that get deactivated, especially those responsible for logical/rational thought.
Freud one of the first to “seriously” study dreams, believing that they represent repressed wishes in disguise, but was completely wrong.
Dreams are very rarely a replay of the days events, but the general emotions of the day very often spill over into the dream.
10. Dreaming as Overnight Therapy
For some benefits of REM sleep, REM sleep itself is just a necessary condition, but also need dreaming!
One of these are emotional benefits.
During dreams, brain completely free of stress triggering noradrenaline.
At same time, emotional and memory centers (amygdala and hippocampus) get highly active.
Thus dreams provide a stress-free environment in which to reprocess old (and possibly painful or emotionally upsetting) memories and integrate them in healthy autobiographical memory.
Sleep heals emotional wounds through dreams.
Experiments even showed that trauma only gets resolved if that trauma shows up in the content of dreams. Just dreaming alone is not enough, needs to be content-specific.
Dreams help separate memory from emotion so that we don’t relive a painful emotion every time we remember something. In patients with PTSD, this is the key problem.
PTSD patient have too high noradrenaline levels. Disrupts their REM sleep and prevents the dreams from healing the memory. But it’s still trying, which leads to recurring nightmares of the traumatic event.
One more emotional benefit of dreaming: Calibrates our understanding of facial expressions so that we can read and interpret other’s emotions.
Particularly, with lack of sleep, misinterpret many facial expressions as hostile.
11. Dream Creativity and Dream Control
Deep sleep strengthens memories, but REM sleep dreaming allows for connecting different memories and novel insights.
Both Mendeleev’s discovery of periodic table as well as Loewi’s of nerve cell communication via synapses and neurotransmitters were dream-inspired.
Similarly in music (e.g. Paul McCartney’s “Yesterday” and Keith Richard’s “Satisfaction”) and literature (Marry Shelley’s “Frankenstein”).
During REM sleep, the usual logical hierarchical structure of memories and associations disappears, and in some way the more unlikely a connection, the better it seems during dreaming phase.
Again, actual content of dreams is important for creative breakthroughs.
Language learning requires coming up with the abstract rules of grammar often just from exposure. Many think that the high amount of REM sleep in young kids makes them so good at it.
A phrase like “sleep on it” exists in a huge number of languages. Sleep subconsciously, through dreams, works on our problems and solves them in novel ways.
[q] “A problem difficult at night is resolved in the morning after the committee of sleep has worked on it.”
- John Steinbeck
Lucid dreaming may be the (current) pinnacle of dream creativity, and may well become the norm for all humans as we evolve further. [highly speculative]
IV. From Sleeping Pills to Society Transformed
12. Things that Go Bump in the Night
Somnambulism, any form of movement during sleep, including sleep walking, actually occurs during dreamless deep NREM sleep.
Insomnia is the most common sleep disorder. Many have it, and many more think they do.
Insomnia is the inability to generate sleep despite giving oneself the adequate opportunity.
Onset and maintenance insomnia depending on whether it’s difficult to fall asleep or stay asleep.
Hyperactive sympathetic nervous system and chronic stress one common cause.
Narcolepsy: Extreme daytime sleepiness, frequent sleep paralysis (waking up but the body taking a while to get out of REM paralysis), and cataplexy (sudden loss of muscle strength leading to fall).
13. iPads, Factory Whistles and Nightcaps
Five key factors have changed how much and how well we sleep:
Electric/LED light
Regularized temperature
Caffeine
Alcohol
Legacy of punching time cards
For much of human history, once the sun set, most activity had to stop. Fire prolonged this a little bit, but effect still very limited. But over last ~200 years, electric light has changed everything.
With nightfall and absence of light, a flood of melatonin would be released, as well as all other sleep inducing processes. This is now thoroughly disrupted.
Artificial light can thus lead to same symptoms as actual onset insomnia.
Blue light (particularly that emitted by many LEDs) particularly bad. Screen in use in evening a major source of bad sleep.
Alcohol often mistakenly considered a sleep aid.
[bq] “Sedation is not sleep."
It doesn’t induce a natural sleep state, more like anesthesia.
Further, alcohol fragments sleep, with regular (often unremembered) wake’s. It also powerfully suppresses REM sleep.
Even just a bit of alcohol in the evening after learning something will have huge detrimental effect on retention. And turns out the same true to almost the same extent on second or third night after learning.
Ambient temperature most under appreciated factor in sleep. Recently have much more stable temperature throughout the day, with much warmer night time temperature.
Core body temp needs to drop by 1C for successful sleep. It’s easier to sleep in a room that’s too cold rather than too hot.
Gently heating hands or feel can actually encourage blood to rise to their surface and cool down core body temperature, leading to better sleep.
Enforced awakening, first in the form of factory whistle, later the more personal alarm clock, also dramatically changed our sleep. Actually a strong trigger for fight-or-flight response. Having that daily (with snooze button sometimes multiple times) quickly build up to chronic stress levels.
Waking up at the same time every day a good way to gradually avoid dependance on alarm clock.
14. Hurting and Helping Your Sleep
Sleeping pills do not actually induce natural sleep state, they just sedate. They also often leave groggy state the next morning, leading to consumption of more stimulants, which worsens the cycle.
Not only that, but use of sleeping pills even in moderate doses linked to higher all-cause mortality and cancer (up to 30-60%)!
Most effective actual treatment at the moment: Cognitive behavioral therapy for insomnia, CBT-I. Several week program with special therapist. Obvious steps include reduction of caffeine/alcohol and better sleep hygiene. But also less obvious interventions.
Exercise improves sleep, but surprisingly not much on a daily level, more on a long term scale.
Strong caloric restriction makes it harder to fall asleep.
15. Sleep and Society
WHO labels societal lack of sleep as global health epidemic.
Every second adult in developed nations does not get enough sleep.
Lack of sleep leads to huge productivity loss in workplace, yet “we overvalue employees that undervalue sleep.”
[bq] “Arrogance in many business cultures focused on the uselessness of sleep.”
[bq] “This mentality has persisted, in part, because certain business leaders mistakenly believe that time on task equates with task completion and productivity.” [p.297f]
Studies show that on national level lack of sleep costs developed nations 2-3% of their GDP!
[bq] “Why try to boil a pot of water on medium heat when you could do so in half the time on high?”
Sleep deprived people don’t perceive themselves as less productive/effective.
[NFW: Dunning–Kruger effect?]
Due to importance of sleep for emotional functioning, leadership rapidly deteriorates with insufficient sleep.
Sleep deprivation still used as a torture method in many countries. But besides the ethical issues, it’s unlikely to achieve its goal of information retrieval since the sleep deprived subject is incapable of proper memory.
Public schools start extremely early. Kids and teenagers often have to get up as early as 5:30am. This is made worse by their shifted circadian rhythm. As if asking and adult to get up at 3am every day, be an effective learner and emotionally stable classmate. Absolute nonsense. Also most susceptible stage in life to develop chronic mental illness.
Find that total sleep time is one of the strongest indicators for good school performance and high IQ.
Leading cause of death in teenagers are car accidents. See dramatic reduction of these when shift to later school start times.
Lack of sleep also happen to have almost identical symptoms to ADHD and might be the true cause of ADHD epidemic. Ironically ADHD drugs are powerful stimulants that further harm sleep.
The risk of medical error or even patient’s death is strongly correlated with the amount of sleep a doctor had in the 24 hours prior to a procedure. Before the next procedure, should ask the doctor how much he slept.
Some large scale tragedies, like Chernobyl and the Exxon Valdez oil tanker, were caused by lack of sleep.
16. A New Vision for Sleep in the 21st Century
Thermostats and lights that are connected via deep learning to personal sleep trackers could lean what out optimal sleep environment is and automate our house accordingly.
Could do the same in offices.
Schools provide some sex, dietary, and exercise education, but currently no sleep education. Should change that!
Insurance Aetna provides bonuses to people who sleep enough based on sleep tracker data. [p. 333]
Flexible work times with a core window (say noon to 3pm) need to become the norm.
Finally at highest level, need to educate policy makers and society better about importance of sleep.