Why We Sleep by Matthew Walker: Summary and Notes

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Sleep enhances our ability to learn, memorize, and make logical decisions. It recalibrates our emotions, restocks our immune system, fine-tunes our metabolism, and regulates our appetite. Lack of sleep is associated with higher rates of depression, anxiety, immune system failure, stroke, heart failure, cancer, dementia, skin problems, and overeating.

Why We Sleep Summary

This is my book summary of Why We Sleep by Matthew Walker. My notes include quotes, big ideas, and important lessons from the book.

“Routinely sleeping less than six or seven hours a night demolishes your immune system, more than doubling your risk of cancer. Insufficient sleep is a key lifestyle factor determining whether or not you will develop Alzheimer’s disease.”
Inadequate sleep – even moderate reductions for just one week – disrupts blood sugar levels so profoundly that you would be classified as pre-diabetic. Sleep disruption further contributes to all major psychiatric conditions, including depression, anxiety, and suicidality.
Too little sleep swells concentrations of a hormone that makes you feel hungry while suppressing a companion hormone that otherwise signals food satisfaction. Despite being full, you still want to eat more.
“The shorter you sleep, the shorter your life span.”
Two processes determine when we feel sleepy – a 24-hour circadian rhythm where the body is naturally awake for 12-16 hours and a chemical called adenosine that slowly builds up starting from the moment we wake and continues to build until it makes us feel very sleepy at night.
Caffeine temporarily blunts the feeling of adenosine, but not the accumulation of it. This is why, once the liver flushes out the caffeine, you experience a crash because you feel the effects of all that built up adenosine at once.
Everyone has a different circadian rhythm. For some people, their peak of wakefulness arrives early in the day, and their sleepiness trough arrives early at night. These are “morning types,” and make up about 40% of the population. These people tend to wake around dawn and are happy to do so and function optimally at this time of day.
Other people are “evening types” and account for about 30% of the population. These people prefer going to bed late and wake up late the following morning.
The remaining 30% of the population is somewhere between a morning and evening type.
Your “type,” i.e. whether you’re a morning or evening person, is known as your chronotype and it’s largely determined by genetics. If you’re a night owl, it’s likely that one or both of your parents is a night owl.
Sadly, society treats night owls as lazy since they don’t like to wake up until later in the morning. In addition, society’s work schedule favors morning types. Evening types are often forced into an unnatural sleep-wake rhythm to meet a certain work schedule, thus evening types are more often sleep-deprived.
“You may be wondering why Mother Nature would program this variability across people. As a social species, should we not all be synchronized and therefore awake at the same time to promote maximal human interactions? Perhaps not. Humans likely evolved to co-sleep as families or even whole tribes, not alone or as couples. The benefits of such genetically programmed variation in sleep/wake timing preferences can be understood. The night owls in the group would not be doing to sleep until one or two a.m. , and not waking until nine or ten a.m. The morning larks, on the other hand, would have retired for the night at nine p.m. and woken at five a.m. Consequently, the group as a whole is only collectively vulnerable (i.e., every person asleep) for just four rather than eight hours, despite everyone still getting the chance for eight hours of sleep. That’s potentially a 50 percent increase in survival fitness.”
In terms of information processing, the wake state is for reception – experiencing events and learning about the world around you, NREM sleep is for reflection – storing and strengthening the raw data you encountered throughout the day, and REM sleep is for integration – interconnecting that raw data with all past experiences, and in doing so, building an even more accurate model of how the world works, including innovative insights and problem-solving abilities.
In evolutionary terms, a biphasal sleep pattern is most natural for humans, i.e. sleeping 8 hours at night, then taking a 30 minute to 1 hour nap in the mid-afternoon when the human body naturally begins to feel a bit tired.
In locations around the world where people live the longest, naps are common.
Melatonin cycles change as we age. Teenagers actually have later cycles, which means they don’t get tired until 11 or midnight, unlike most adults who tend to get tired earlier. This is why calling teenagers “lazy” for sleeping in late may not be accurate; they’re simply adhering to their natural melatonin levels.
“Sleep enhances your memory and makes you more creative. It makes you look more attractive. It keeps you slim and lowers food cravings. It protects you from cancer and dementia. It wards off colds and the flu. It lowers your risk of heart attacks and stroke, not to mention diabetes. It helps you feel happier, less depressed, and less anxious.”
Sleep before learning something new refreshes our ability to initially make new memories. Sleep after learning something new enhances memory retention.
More sleep is associated with improved athletic performance. One study on professional basketball player Andre Iguodala found that several of his measurable statistics on court improved when he got eight hours of sleep per night compared to less than eight hours of sleep.
Insufficient sleep is strongly associated with Alzheimer’s disease.
“The less you sleep, the more you are likely to eat. In addition, your body becomes unable to manage those calories effectively, especially the concentrations of sugar in your blood. In these two ways, sleeping less than seven or eight hours a night will increase your probability of gaining weight, being overweight, or being obese, and significantly increases your likelihood of developing type 2 diabetes.”
Dr. Even Van Cauter from the University of Chicago conducted a study to measure how lack of sleep correlates with overeating: “Participants in this experiment underwent two different conditions, acting as their own baseline control: four nights of eight and a half hours’ time in bed, and four nights of four and a half hour’s time in bed. Each day, participants were limited to the same level of physical activity under both conditions. Each day, they were given free access to food, and the researchers meticulously counted the difference in calorie consumption between the two experimental manipulations. When short sleeping, the very same individuals ate 300 calories more each day – or well over 1,000 calories before the end of the experiment – compared to when they were routinely getting a full night of sleep.”
“Similar changes occur if you give people five to six hours of sleep over a ten-day period. Scale that up to a working year, and assuming one month of vacation in which sleep miraculously becomes abundant, and you will still have consumed more than 70,000 extra calories. Based on caloric estimates, that would cause 10 to 15 pounds of weight gain a year, each and every year.”
“Weight gain caused by short sleep is not just a matter of eating more, but also a change in what you binge eat. Looking across the different studies, Van Cauter noticed that cravings for sweets (e.g., cookies, chocolate, and ice cream), heavy-hitting carbohydrate-rich foods (e.g., bread and pasta), and salty snacks (e.g., potato chips and pretzels) all increased by 30 to 40 percent when sleep was reduced by several hours each night.”
Sleep deprivation is linked to infertility or sub-fertility.
Sleep improves physical appearance. In one study, groups of individuals who were given 8 hours of sleep the night before were rated as more attractive compared to another group of individuals who were only given 5 hours of sleep the night before.
“Sleep fights against infection and sickness by deploying all manner of weaponry within your immune arsenal, cladding you with protection. When you do fall ill, the immune system actively stimulates the sleep system, demanding more bed rest to help reinforce the war effort. Reduce sleep even for a single night, and that invisible suit of immune resilience is rudely stripped from your body.”
Natural killer cells help fight tumor cells – “Natural killer cells will effectively punch a hole in the outer surface of these cancerous cells and inject a protein that can destroy the malignancy. What you want, therefore, is a virile set of these James Bond-like immune cells at all times. That is precisely what you don’t have when sleeping too little.”
“Dr. Michael Irwin at the University of California, Los Angeles, has performed landmark studies revealing just how quickly and comprehensively a brief dose of short sleep can affect your cancer-fighting immune cells. Examining healthy young men, Irwin demonstrated that a single night of four hours of sleep – such as going to bed at three a.m. and waking up at seven a.m. – swept away 70 percent of the natural killer cells circulating in the immune system, relative to a full eight-hour night of sleep. That is a dramatic state of immune deficiency to find yourself facing, and it happens quickly, after essentially one “bad night” of sleep. You could well imagine the enfeebled state of your cancer-fighting immune armory after a week of short sleep, let alone months or even years.”
Sleeping pills do not provide the same restorative immune benefits as natural sleep. Effectively, sleeping pills sedate you, which helps you fall asleep faster, but the sleep you get is poor quality.
“If you compare natural, deep-sleep brainwave activity to that induced by modern-day sleeping pills, such as zolpidem (brand name Ambien) or eszopiclone (brand name Lunesta), the electrical signature, or quality is deficient. The electrical type of “sleep” these drugs produce is lacking in the largest, deepest brainwaves. Adding to this state of affairs are a number of unwanted side effects, including next-day grogginess, daytime forgetfulness, performing actions at night of which you are not conscious (or at least have partial amnesia of in the morning), and slowed reaction times during the day that can impact motor skills, such as driving.”
“Another deeply unpleasant feature of sleeping pills is rebound insomnia. When individuals stop taking these medications, they frequently suffer far worse sleep, sometimes even worse than the poor sleep that led them to seek out sleeping pills to begin with. The cause of rebound insomnia is a type of dependency in which the brain alters its balance of receptors as a reaction to the increased drug dose, trying to become somewhat less sensitive as a way of countering the foreign chemical within the brain. This is also known as drug tolerance. But when the drug is stopped, there is a withdrawal process, part of which involves an unpleasant spike in insomnia severity.”
“Employees win financially when sleep times increase. Those who sleep more earn more money, on average, as economists Matthew Gibson and Jeffrey Shrader discovered when analyzing workers and their pay across the United States. They examined townships of very similar socio-educational and professional standing within the same time zone, but at very far western and eastern edges of these zones that receive significantly different amounts of daylight hours. Workers in the far western locations obtained more sunlight later into the evening, and consequently went to bed an hour later, on average, than those in the far eastern locations…
…However, all workers in both regions had to wake up at the same time each morning, since they were all in the same time zone and on the same schedule. Therefore, western-dwelling workers in that time zone had less sleep opportunity than the eastern-dwelling workers…
…Factoring out many other potential factors and influences (e.g., regional affluence, house prices, cost of living, etc.), they found that an hour of extra sleep still returned significantly higher wages in those eastern locations, somewhere in the region of 4 to 5 percent. You may sniff at that return on the investment of sixty minutes of sleep, but it’s not trivia. The average pay raise in the US is around 2.6 percent. Most people are strongly motivated to get that raise, and are upset when they don’t. Imagine almost doubling that pay raise – not by working more hours, but by getting more sleep!”
Matthew Walker provides twelve tips for healthy sleep:
“1. Stick to a sleep schedule. Go to bed and wake up at the same time each day. As creatures of habits, people have a hard time adjusting to changes in sleep patterns. Sleeping later on weekends won’t fully make up for a lack of sleep during the week and will make it harder to wake up early on Monday morning. Set an alarm for bedtime. Often we set an alarm for when it’s time to wake up but fail to do so for when it’s time to go to sleep. If there is only one piece of advice you remember and take from these twelve tips, this should be it.”
“2. Exercise is great, but not too late in the day. Try to exercise at least thirty minutes on most days but not later than two to three hours before your bedtime.”
“3. Avoid caffeine and nicotine. Coffee, colas, certain teas, and chocolate contain the stimulant caffeine, and its effects can take as long as eight hours to wear off fully. Therefore, a cup of coffee in the late afternoon can make it hard for you to fall asleep at night. Nicotine is also a stimulate, often causing smokers to sleep only very lightly. In addition, smokers often wake up too early in the morning because of nicotine withdrawal.”
“4. Avoid alcoholic drinks before bed. Having a nightcap or alcoholic beverage before sleep may help you relax, but heavy use robs you of REM sleep, keeping you in the lighter stages of sleep. Heavy alcohol ingestion also may contribute to impairment in breathing at night. You also tend to wake up in the middle of the night when the effects of the alcohol have worn off.”
“5. Avoid large meals and beverages late at night. A light snack is okay, but a large meal can cause indigestion, which interferes with sleep. Drinking too many fluids at night can cause frequent awakenings to urinate.”
“6. If possible, avoid medications that delay or disrupt your sleep. Some commonly prescribed heart, blood pressure, or asthma medications, as well as some over-the-counter and herbal remedies for coughs, colds, or allergies, can disrupt sleep patterns. If you have trouble sleeping, talk to your health care provider or pharmacist to see whether any drugs you’re taking might be contributing to your insomnia and ask whether they can be taken at other times during the day or early in the evening.”
“7. Don’t take naps after 3 p.m. Naps can help make up for lost sleep, but late afternoon naps can make it harder to fall asleep at night.”
“8. Relax before bed. Don’t overschedule your day so that no time is left for unwinding. A relaxing activity, such as reading or listening to music, should be part of your bedtime ritual.”
“9. Take a hot bath before bed. The drop in body temperature after getting out of the bath may help you feel sleepy, and the bath can help you relax and slow down so you’re more ready to sleep.”
“10. Dark bedroom, cool bedroom, gadget-free bedroom. Get rid of anything in your bedroom that might distract you from sleep, such as noises, bright lights, an uncomfortable bed, or warm temperatures. You sleep better if the temperature in the room is kept on the cool side. A TV, cell phone, or computer in the bedroom can be a distraction and deprive you of needed sleep. Having a comfortable mattress and pillow can help promote a good night’s sleep. Individuals who have insomnia often watch the clock. Turn the clock’s face out of view so you don’t worry about the time while you’re trying to fall asleep.”
“11. Have the right sunlight exposure. Daylight is key to regulating daily sleep patterns. Try to get outside in natural sunlight for at least thirty minutes each day. If possible, wake up with the sun or use very bright lights in the morning. Sleep experts recommend that, if you have problems falling asleep, you should get an hour of exposure to morning sunlight and turn down the lights before bedtime.”
“12. Don’t lie in bed awake. If you find yourself still awake after staying in bed for more than twenty minutes or if you are starting to feel anxious or worried, get up and do some relaxing activity until you feel sleepy. The anxiety of not being able to sleep can make it harder to fall asleep.”

Recommendation for Better Sleep

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