Why We Sleep By Mathew Walker

Unlocking the Power of Sleep and Dreams

Published or Updated on 
February 4, 2021


Note: Many claims in the book have been debunked. While I still think it is a very good piece of writing with good tips for better health overall, I wouldn’t take everything that the book says at face value (one never should do this). Read this article for more information.

Notes & Highlights

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. Short sleeping increases the likelihood of your coronary arteries becoming blocked and brittle, setting you on a path toward cardiovascular disease, stroke, and congestive heart failure.

The shorter your sleep, the shorter your life span. The old maxim “I’ll sleep when I’m dead” is therefore unfortunate.

“If sleep does not serve an absolutely vital function, then it is the biggest mistake the evolutionary process has ever made.”

Dreaming provides a unique suite of benefits to all species fortunate enough to experience it, humans included. Among these gifts are a consoling neurochemical bath that mollifies painful memories and a virtual reality space in which the brain melds past and present knowledge, inspiring creativity.

It is no coincidence that the likelihood of breaking an Olympic record has been clearly tied to time of day, being maximal at the natural peak of the human circadian rhythm in the early afternoon. Even the timing of births and deaths demonstrates circadian rhythmicity due to the marked swings in key life-dependent metabolic, cardiovascular, temperatura, and hormonal processes that this pace-maker controls.

Sunlight acts like a manipulating finger and thumb on the side-dial of an imprecise wristwatch. The light of the sun methodically resets our inaccurate internal timepiece each and every day, “winding” us back to precisely, not approximately, twenty-four hours.

The prefrontal cortex controls high-level thought and logical reasoning, and helps keep our emotions in check. When a night owl is forced to wake up too early, their prefrontal cortex remains in a disabled, “offline”, state.

Melatonin simply provides the official instruction to commence the event of sleep, but does not participate in the sleep race itself.

The longer you are awake, the more adenosine will accumulate. Think of adenosine as a chemical barometer that continuously registers the amount of elapsed time since you woke up this morning.

Caffeine is not a food supplement. Rather, caffeine 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 last statement is a bit inaccurate but I get the point. Sugar, wheat, etc. are usually more traded than cofee… ]]

Levels of circulating caffeine peak approximately thirty minutes after oral administration. What is problematic, though, is the persistence of caffeine in your system. In pharmacology, we use the term “half-life” when discussing a drug’s efficacy.

Caffeine has an average half-life of five to seven hours. Let’s say that you have a cup of coffee after your evening dinner, around 7.30 p.m. This means that by 1.30 a.m., 50 percent of that caffeine may still be active and circulating throughout your brain tissue.

Most people do not realize how long it takes to overcome a single dose of caffeine, and therefore fail to make the link between the bad night of sleep we wake from in the morning and the cup of coffee we had ten hours earlier with dinner.

Your brain, it seems, is still capable of logging time with quite remarkable precision while asleep.

The uneven back-and-forth interplay between NREM and REM sleep is necessary to elegantly remodel and update our neural circuits at night, and in doing so manage the finite storage space within the brain.

A key function of deep NREM sleep, which predominates early in the night, is to do the work of weeding out and removing unnecessary neural connections. In contract, the dreaming stage of REM sleep, which prevails later in the night, plays a role in strengthening those connections.

Pruning of connections becomes the order of the day, or should I say, night. Enter the sculpting hand of deep NREM sleep.

Deep sleep may be a driving force of brain maturation, not the other way around.

REM sleep can therefore be considered as a state characterized by strong activation in visual, emotional, and autobiographical memory regions of the brain, yet a relative deactivation in regions that control rational thought.

We have found two core benefits of REM sleep. Both functional benefits require not just that you have REM sleep, but that you dream, and dream about specific things. REM sleep is necessary, but REM sleep alone is not sufficient. Dreams are not the heat of the lightbulb —they are not a by-product.

[Deep sleep]

This feature, termed “atonia” (an absence of tone, referring here to the muscles), is instigated by a powerful disabling signal that is transmitted down the full length of your spinal cord from your brain stem.

Why did life ever bother to wake up? Considering how biologically damaging the state of wakefulness can often be, that is the true evolutionary puzzle here, not sleep.

Only birds and mammals, which appeared later in the evolutionary timeline of the animal kingdom, have full-blown REM sleep.

When a theme repeats itself in evolution, and independently across unrelated lineages, it often signals a fundamental need.

Take cetaceans, such as dolphins and whales, for example. Their sleep, of which there is only NREM, can be unihemispheric, meaning they will sleep with half a brain at a time! One half of the brain must always stay awake to maintain life-necessary movement in the aquatic environment.

Individuals who are deliberately fasting will sleep less as the brain is tricked into thinking that food has suddenly become scarce.

[In my experience this hasn’t been the case. Wonder what are the implications and the research backing this. ]

The practice of biphasic sleep is not cultural in origin, however. It is deeply biological. All humans, irrespective of culture or geographical location, have a genetically hardwired dip in alertness that occurs in the midafternoon hours.

[This is counters the previous assertion that Olympic records are more prone to be broken in the afternoon, because of our circadian rhythm]

Alcohol is one of the most powerful suppressor of REM sleep that we know of.

sleep is the universal health care provider: whatever the physical or mental ailment, sleep has a prescription it can dispense.

Even daytime naps as short as twenty minutes can offer a memory consolidation advantage, so long as they contain enough NREM sleep.

Slow rocking (the bed) increased the depth of deep sleep, boosted the quality of slow brainwaves, and more than doubled the number of sleep spindles. …the findings offer a scientific explanation for the ancient practice of rocking a child back and forth in one’s arms, or in a crib, inducing a deep sleep.

[I always say that sleeping on a hammock is awesome.]

Practice, with sleep, makes perfect.

Obtain anything less than eight hours of sleep a night, and especially less than six hours a night, and the following happens: time to physical exhaustion drops by 10 to 30 percent, and aerobic output is significantly reduced. Similar impairments are observed in limb extension force and vertical jump height, together with decreases in peak and sustained muscle strength.

Sleep provides a nighttime theater in which your brain tests out and builds connection between vast stores of information. This task is accomplished using a bizarre algorithm that is biased toward seeking out the most distant, nonobvious associations, rather like a backward Google search.

The recycle rate of a human being is around sixteen hours. After around sixteen hours of being awak, the brain begins to fail. Humans need more than seven hours of sleep each night to maintain cognitive performance. After ten days of just seven hours of sleep, the brain is as dysfunctional as it would be after going without sleep for twenty-four hours. Three full nights of recovery sleep (i.e. more nights than a weekend) are insufficient to restore performance back to normal levels after a week of short sleeping. Finally, the human mind cannot accurately sense how sleep deprived it is when sleep-deprived.

“The best bridge between despair and hope is a good night’s sleep.”

Wakefulness is low-level brain damage, while sleep is neurological sanitation.

Without sufficient sleep, amyloid plaques build up in the brain, especially in deep-sleep-generating regions, attacking and degrading them. The loss of deep NREM sleep caused by this assault therefore lessens the ability to remove amyloid from the brain at night, resulting in greater amyloid deposition. More amyloid, less deep sleep, less deep sleep, more amyloid, and so on and so forth.

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.

When you are not getting enough sleep, the body becomes especially stingy about giving up fat. Instead, muscle mass is depleted while fat is retained. Lean and toned is unlikely to be the outcome of dieting when you are cutting sleep short. The latter is counterproductive of the former.

Journaling your waking thoughts, feelings and concerns has a proven mental health benefit, and the same appears true of your dreams. A meaningful, psychologically healthy life is an examined one, as Socrates so often declared. Nevertheless, the psychoanalytic method built on Freudian theory is nonscientific and holds no repeating, reliable, or systematic power for decoding dreams. This, people must be made aware of.

It is sleep that builds connections between distantly related informational elements that are not obvious in the light of the waking day.

Like an insightful interviewer, dreaming takes the approach of interrogating our recent autobiographical experience and skillfully positioning it within the context of past experiences and accomplishments, building a rich tapestry of meaning.

Humans are predominantly visual creatures. More than a third of our brain is devoted to processing visual information.

Alcohol is one of the most powerful suppressors of REM sleep that we know of. When the body metabolizes alcohol it produces by-product chemicals called aldehydes and ketones. The aldehydes in particular will block the brain’s ability to generate REM sleep.

To successfully initiate sleep, your core temperature needs to decrease by 2 to 3 degrees Fahrenheit, or about 1 degree Celsius. For this reason, you will always find it easier to fall asleep in a room that is too cold than too hot, since a room that is too cold is at least dragging your brain and body in the correct (downward) temperature direction for sleep.

..the hot bath invites blood to the surface of your skin, giving you that flushed appearance. When you get out of the batch, those dilated blood vessles on the surface quickly help radiate out inner heat, and your core body temperature plummets. Consequently, you fall asleep more quickly because your core is colder. Hot baths prior to bed can also induce 10 to 15 percent more deep NREM sleep in healthy adults.

What you eat also appears to have some impact on your nighttime sleep. Eating a high-carbohydrate, low-fat diete for two days decreases the amount of deep NREM sleep at night, but increases the amount of REM sleep dreaming, relative to a two-day diet low in carbohydrates and high in fat. In a carefully controlled study of healthy adult individuals, a four-day diet high in sugar and other carbohydrates, but low in fiber, resulted in deep NREM sleep and more awakenings at night.

Those how sleep more earn more money, on average.

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