Why We Sleep - Matthew Walker Flashcards
Part One - This Thing Called Sleep
Routinely sleeping less than 6 or 7 hours a night demolishes your immune system, more than doubling your risk of cancer.
Too little sleep swells concentrations of a hormone that makes you feel hungry while suppressing a companion hormone that otherwise signals food satisfaction.
Leptin is the hormone that decreases your appetite. Ghrelin is the hormone that increases your appetite.
Swell = become or make greater in intensity, number, amount, or volume; grow, enlarge, increase
Human beings are the only species that will deliberately deprive themselves of sleep without legitimate gain.
Society’s apathy toward sleep has been caused by the historic failure of science to explain why we need it.
The perseverance of sleep throughout evolution means there must be tremendous benefits that far outweigh all of the obvious hazards and detriments.
We sleep for a rich litany of functions.
There does not seem to be one major organ within the body that isn’t optimally enhanced by sleep.
The physical and mental impairments caused by one night of bad sleep dwarf those caused by an equivalent absence of food or exercise.
A living organism keeps its own time and is not slave to the sun’s rhythmic commands.
Humans generate their own circadian rhythm in the absence of external light from the sun.
Often referred to as the “body clock”, the circadian rhythm is a cycle that tells our bodies when to sleep, rise, eat - regulating many physiological processes.
It is approximately one day in length but not precisely one day.
The 24 hour biological clock sitting in the middle of your brain is the called the suprachiasmatic (pronounced soo-pra-kai-as-MAT-ik) nucleus.
This tiny clock controls when you want to be awake and asleep and not the other way around.
Your circadian rhythm will march up and down every twenty four hours irrespective of whether you have slept or not.
Chronotype - a person’s natural inclination with regard to the times of day when they prefer to sleep or when they are most alert or energetic.
Morning larks will chastise night owls on the erroneous assumption that such preferences are a choice. However, night owls are not owls by choice. They are bound to a delayed schedule by unavoidable DNA hardwiring.
Your suprachiastic nucleus communicates its repeating signal of night and day to your brain and body using a circulating messenger called melatonin.
Melatonin is also called the hormone of darkness because it is released at night.
Melatonin only helps regulate the timing of when sleep occurs by systematically signalling darkness through the organism; it has little influence of the generation of sleep itself.
There is a significant sleep placebo effect in melatonin. The placebo effect is the most reliable effect in all of pharmacology.
Placebo effect - psychological phenomenon, in which the recipient perceives an improvement in condition due to personal expectations, rather than the treatment itself.
Jet lag places a torturous physiological strain on the brain, and a deep biological stress upon the cells, organs and major systems of the body.
There are 2 factors determining wake and sleep:
1/ Circadian rhythm
2/ Sleep pressure
Sleep pressure accumulates with the release of a chemical called adenosine. The longer you are awake, the more will accumulate. Think of adenosine as a chemical barometer that continuously registers the amount of elapsed time since you woke up this morning.
When adenosine concentrations peak, an irresistible urge for slumber will take hold. It happens to most people after 12 to 16 hours of being awake.
You can artificially mute the sleep signal of adenosine by using a chemical that makes you feel more alert and awake: CAFFEINE.
Caffeine is not a food supplement. It is the most widely used (and abused) psychoactive stimulant in the world.
It is the second most traded commodity on the planet, after oil.
The consumption of caffeine represents one of the longest and largest unsupervised drug studies ever conducted on the human race, perhaps rivalled only by alcohol.
Caffeine tricks you into feeling alert and awake, despite the high levels of adenosine that would otherwise seduce you into sleep.
Caffeine has an average half-life of 5 to 7 hours i.e. 50% of the caffeine may still be active and circulating throughout your brain tissue 5 to 7 hours later.
Most people do not realise how long it takes to overcome a single dose of caffeine.
De-caffeinated does not mean non-caffeinated. It just contains 15 to 30 per cent compared to the regular dose.
For the entire time that caffeine is in your system, the sleepiness chemical it blocks (adenosine) nevertheless continues to build up. Once your liver dismantles the caffeine, you feel a vicious backlash (the caffeine crash).
It is worth pointing out that caffeine is a stimulant drug. It is the only addictive substance we readily give to our children.
2 cups of tea = 1 cup of coffee. There are 20mg of caffeine in your average 100g of brewed tea compared to 40mg in the same amount of black filter coffee.
2 colas = 1 tea. 100g of your average cola contains just 8mg of caffeine - although for reduced sugar varieties that number goes up to 15mg.
The circadian rhythm and sleep pressure do not communicate with each other. They are distinct and separate systems.
During sleep a mass evacuation gets under way as the brain has the chance to degrade and remove the day’s adenosine. After 8 hours, the purge is complete; just as this process is ending, your circadian activity begins to rouse and its energising influence approaches. When these two processes trade places we naturally wake up.
Following that full night of sleep, you are ready to face another 16 hours of wakefulness with physical vigour and sharp brain function.
Humans are diurnal so we have a preference for being awake throughout the day and sleeping at night.
Your brain is capable of logging time with quite remarkable precision while asleep.
Rapid eye movement (REM) sleep - the stage in which humans principally dream.
Humans cycle through 2 completely different types of sleep based on their defining ocular features: NREM and REM sleep.
NREM and REM sleep battle for brain domination across the night. The cerebral war between the 2 is won and lost every 90 minutes.
Short-changing the brain of either REM or NREM sleep - both of which serve critical, though different, brain and body functions - results in a myriad of physical and mental ill health.
Deep NREM sleep is one of the most epic displays of neural collaboration that we know of. The rhythmic deep NREM slow-wave sleep is a highly active, meticulously coordinated state of cerebral unity. Scientists have been forced to abandon any cursory notions of deep sleep as a state of dull stupor.
REM sleep has also been called paradoxical sleep: a brain that appears awake, yet a body that is clearly asleep. It is often impossible to distinguish REM sleep from wakefulness using just electrical brainwave activity.
When it comes to information processing, think of the wake state principally as reception, NREM sleep as reflection, and REM sleep as integration.
Why did evolution decide to outlaw muscle activity during REM sleep? The brain paralyses the body so the mind can dream safely.
The geneticist Theodosius Dobzhansky once said, “Nothing in biology makes sense except in light of evolution”.
Like other rudimentary features, such as DNA, sleep has remained a common bond uniting every creature in the animal kingdom.
It’s important to note that regardless of the amount of recovery opportunity, the brain never comes close to getting back all the sleep it has lost. Sleep is non-negotiable.
Our circadian biology, and the insatiable early morning demands of a post-industrial way of life, denies us the sleep we vitally need.
All humans, irrespective of culture or geographical location, have a genetically hardwired dip in alertness that occurs in the mid afternoon hours.
Biphasic sleep = a longer bout of continuous sleep at nights, followed by a short mid afternoon nap.
On the island of Ikaria where siestas remain intact, men are nearly 4 times as likely to reach the age of ninety. These napping communities have sometimes been described as the ‘places where people forgot to die’.
REM sleep exquisitely recalibrates and fine-tunes the emotional circuits of the human brain.
The coolheaded ability to regulate our emotions each day - a key to what we call emotional IQ - depends on getting sufficient REM sleep night after night.
The adaptive benefits conferred by complex emotional processing are truly monumental, and so often overlooked.
The second evolutionary contribution that the REM-sleep dreaming state fuels is creativity.
Sleep cycle by sleep cycle, REM sleep helps construct vast associative networks of information within the brain.
In the foetus, REM sleep is vital for promoting brain maturation.
Charged with the herculean task of neuro-architecture -establishing the neural highways and side streets that will engender thoughts, memories, feelings, decisions, and actions - it’s no wonder REM sleep must dominate most, if not all, of early developmental life.
An infant brain without sleep will be a brain ever underconstructed.
Alcohol is one of the most powerful suppressors of REM sleep that we know of.
Studies have now linked alcohol use during pregancy and an increased likelihood of neuropsychiatric illness in the mother’s child, including autism.
The downgrading of the REM-sleep portion, and the upswing in NREM sleep dominance, continues, throughout early and mid childhood. That balance will finally stabilise to an 80/20 split by the late teen years and remain so throughout adulthood.
Adolescents have a less rational version of an adult brain, one that takes more risks and has relatively poor decision-making skills. It takes deep sleep and developmental time to establish the neural maturation that plugs this brain “gap” within the front lobe.
Sadly, neither society nor our parental attitudes are well designed to appreciate or accept that teenagers need more sleep than adults. They are biologically wired to obtain that sleep at a different time from their parents.
“No child needs caffeine.”
The lower an older individual’s sleep efficiency, the higher their mortality risk, the worse their physical health, the more likely they are to suffer from depressions, the less energy they report, and the lower their cognitive function, typified by forgetfulness.
An innocent doze can have a damaging consequence. The early-evening snooze will jettison precious sleep pressure, clearing away the sleepiness power of adenosine that had been steadily building throughout the day.
As individuals get older, their brains do not deteriorate uniformly. Instead, some parts of the brain start losing neutrons much earlier and faster than others - a process called atrophy.
Poor sleep and poor memory in old age are not coincidental but rather significantly interrelated.
Part Two - Why Should You Sleep
Sleep does the following:
Enhances your memory Makes you more creative Makes you look more attractive Keeps you slim Lowers food cravings Protects you from cancer and dementia Wards off colds and the flu Lowers your risk of heart attacks and stroke, not to mention diabetes Makes you feel happier, less depressed and less anxious
Failed by the lack of public education, most of us do not realise how remarkable a panacea sleep truly is.
Sleep is the universal health care provider
Sleep is not the absence of wakefulness. It is an exquisitely complex, metabolically active, and deliberately ordered series of unique stages.
The hippocampus offers a short-term information store for accumulating new memories, but it has a limited storage capacity.
Sleep restores the brain’s capacity for learning, making room for new memories.
Memory refreshment is related to lighter, stage 2 NREM sleep, and specifically the short, powerful bursts of electrical activity called sleep spindles. The more sleep spindles an individual obtained during the nap, the greater the restoration of their learning when they woke up.
An electrical transaction occurring in the quiet secrecy of sleep is once that shifts fact-based memories from the temporary storage depot (the hippocampus) to a long-term secure vault (the cortex).
Sleep 6 hours or less and you are short-changing the brain of a learning restoration benefit that is normally performed by sleep spindles.
Sleep is constantly modifying the information architecture of the brain at night.
At every stage of human life, the relationship between NREM sleep and memory solidification is observed.
Like a computer hard drive where some files have become corrupted and inaccessible, sleep offers a recovery service at night.
By boosting the electrical quality of deep-sleep brainwave activity (via transcranial direct current brain stimulation), researchers can double the number of facts that individuals can recall the following day.
Sleep is far more intelligent than we once imagined.
Your brain continues to improve skill memories in the absence of any further practice. It is the number of those wonderful sleep spindles in the last 2 hours of the late morning that are linked with offline memory boost.
Obtain anything less than 8 hours of sleep and the following happens:
Time to physical exhaustion drops by 10 to 30 per cent
Aerobic output is significantly reduced
Peak and sustained muscle strength are decreased
Faster rates of lactic acid buildup
Reductions in blood oxygen saturation
Increases in blood oxygen carbon dioxide
Impaired ability to cool the body through sweating
In ways your waking brain would never attempt, the sleeping brain fuses together disparate sets of knowledge that foster impressive problem-solving abilities.
One brain function that buckles under the smallest dose of sleep deprivation is concentration.
Based on 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 maximising their potential of mind or body due to their blind persistence in sleeping too little.
The recycle rate of a human being is around sixteen hours. After sixteen hours of being awake, the brain begins to fail. Humans need more than 7 hours of sleep each night to maintain cognitive performance.
Neither naps nor caffeine can salvage more complex functions of the brain, including learning, memory, emotional stability, complex reasoning, or decision-making.
There is a very rare collection of individuals who are able to survive on 6 hours of sleep and show minimal impairment. Part of the explanation appears to lie in their genetics, specifically a sub-variant of a gene called BHLHE41.
Why are the emotion centres of the brain so excessively reactive without sleep?
With a full night of plentiful sleep, we have a balanced mix between our emotional gas pedal (amygdala) and brake (prefrontal cortex). Without sleep, however, the strong coupling between these 2 brain regions is lost.
Insufficient sleep does not push the brain into a negative mood state and hold it there. Rather, the under-slept brain swings excessively to both extremes of emotional valence, positive and negative.
There is no major psychiatric condition in which sleep is normal.
“The best bridge between despair and hope is a good night’s sleep”.
Sleep deprivation even impacts the DNA and the learning-related genes in the brain cells of the hippocampus itself. A lack of sleep therefore is a deeply penetrating and corrosive force that enfeebles the memory-making apparatus within your brain.
If you don’t sleep the very first night after learning, you lose the chance to consolidate those memories, even if you get lots of “catch up” sleep thereafter. In terms of memory, then, sleep is not like the bank.
Dementia and cancer are both related to inadequate sleep.
It’s curious that Margaret Thatcher and Ronald Reagan - both vocal and proud of sleeping only about 4 to 5 hours per night - both went on to develop the ruthless disease that is Alzheimer’s.
Many people think nothing of losing an hour of sleep for a single night, believing it to be trivial and inconsequential. It is anything but.
From a metabolic perspective, sleep deprivation causes loss of hunger control. A sleep-deprived body will cry famine in the midst of plenty.
Ample sleep can restore a system of impulse control within your brain, putting the appropriate brakes on potentially excessive eating.
The faces of those after one night of short sleep were rated as looking more fatigued, less healthy, and significantly less attractive, compared with the appealing images of the same individuals after they had slept a full 8 hours.
An intimate and bidirectional association exists between your sleep and your immune system.
Sleep deprivation boosts levels of cells which promote cancer growth.
Chronic sleep loss erodes the very essence of biological life itself: your genetic code and the structures that encapsulate it.
Anything that causes a wobble in gene stability can have consequences. When you do not lavish these DNA segments with enough sleep, they will not translate their instructional code into action, and give the brain and body what they need.
If the telomeres at the end of your chromosomes become damaged, your DNA spirals become exposed and your now vulnerable genetic code cannot operate properly, like a fraying shoelace without a tip.
The less sleep an individual obtains, or the worse the quality of sleep, the more damaged the telomeres of that individual’s chromosomes.
2 individuals of the same chronological age would not appear to be of the same biological age on the basis of their telomere health if one was routinely sleeping five hours a night while the other was sleeping 7 hours a night. The latter would appear “younger”.
Not enough sleep significantly modifies your gene transcriptome - the very essence of you as defined biologically by your DNA.
Besides being the greatest creative aphrodisiac, sleep also affects our every waking moment, dictates our social rhythm, and even mediates our negative moods. Be as religious and disciplined about your sleep as you are about your work. We tend to wear our ability to get by on little sleep as some sort of badge of honor that validates our work ethic. But what it really is is a profound failure of self-respect and of priorities.
Part 3 How and Why We Dream
The prefrontal cortex acts like the CEO of the brain. This region manages rational thought and logical decision-making, sending “top-down” instructions to your more primitive deep-brain enters, such as those instigating emotions. It is temporarily ousted each time you enter a state of REM sleep.
Freud wrested dreams from the ownership of celestial beings, and from the anatomically unclear location of the soul. In his seminal book The Interpretation of Dreams (1899), he situated the dream unquestionably within the brain.
Concentrations of key stress-related chemical called noradrenaline are completely shut off within your brain when you enter the dreaming sleep state.
We have REM-sleep dreaming to thank us for the palliative dissolving of emotion from experience.
It’s not time that heals. Instead it is time spent in dream sleep that provides emotional convalescence.
We can think of REM sleep like a master piano tuner, one that readjusts the brain’s emotional instrumentation at night to pitch-perfect precision, so that when you wake up the next morning, you can discern overt and subtly covert micro-expressions with exactitude.
During the dreaming sleep state, your brain will cogitate vast swaths of acquired knowledge, and then extract overarching rules and commonalities - “the gist”.
REM sleep is the difference between knowledge (retention of individual facts) and wisdom (knowing what they all mean when you fit them together). Or said more simply, learning versus comprehension.
Narcolepsy is a neurological disorder that affects the control of sleep and wakefulness. People with narcolepsy experience excessive daytime sleepiness and intermittent, uncontrollable episodes of falling asleep during the daytime.
Some people confuse time slept with sleep opportunity time.
Weak immune systems are a known consequence of insufficient sleep.
5 key factors have powerfully changed how much and how well we sleep:
1/ Constant electric light as well as LED light 2/ Regularised temperature 3/ Caffeine 4/ Alcohol 5/ A legacy of punching time cards
Loss of daylight informs our suprachiastic nucleus that nightime is now in session; time to release the brake pedal on our pineal gland, allowing it to unleash vast quantities of melatonin that signal to our brains and bodies that darkness has arrived and it is time for bed. Electric light put an end to this natural order of things.
Compared to reading a book, reading an iPad suppressed melatonin release by over 50 per cent at night (up to 3 hours).
The frontal lobe region of the human brain helps control our impulses and restrains our behaviour. Alcohol immobilises that part of our brain first.
Alcohol sedates you out of wakefulness but it does not induce natural sleep; rather it is akin to a light form of anaesthesia.
Alcohol fragments sleep, littering the night with brief awakenings. It is therefore not continuous or restorative.
People fail to link the effects of alcohol and next-day exhaustion.
People consuming even moderate amounts of alcohol in the afternoon and/or evening are depriving themselves of dream sleep.
The brain has a non-negotiable requirement for sleep the first night after learning for the purposes of memory processing.
Hot baths prior to bed can induce 10 to 15 per cent more deep NREM sleep in healthy adults.
No other species demonstrates this unnatural act of prematurely and artificially terminating sleep.
When sleep is poor the night before, exercise intensity and duration are far worse the following day.
Sleep will boost your fitness and energy, setting in motion a positive, self-sustaining cycle of improved physical activity (and mental health).
Business leaders mistakenly believe that time on-task equates with task completion and productivity. It is a misguided and expensive fallacy.
Sleepy employees are unproductive employees.
The loud and proud corporate mentality of sleeplessness as the model for success is wrong at every level of analysis we have explored.
If sleep really is rudimentary to learning then increasing sleep time by delaying start times should prove transformative.
“Hamstrung by a privation of sleep”.
Tips for a healthy sleep:
1/ Establish a regular bed-time/sleep schedule
2/ No alcohol or caffeine
3/ Exercise is great but not too late in the day
4/ Avoid large meals and beverages late at night
5/ Don’t take naps after 3pm
6/ Relax before bed. Read a book. Listen to music.
7/ Take a hot bath/shower before bed
8/ Dark bedroom, cool bedroom, gadget-free bedroom
9/ Have the right sunlight exposure the next day
10/ Don’t lie in bed awake.
