Why We Sleep: Unlocking the Power of Sleep and Dreams

by Matthew Walker

Personally,

CHAPTER

It was a revolutionary discovery: de Mairan had shown that a living organism kept its own time, and was not, in fact, slave to the sun’s rhythmic commands. Somewhere within the plant was a twenty-four-hour rhythm generator that could track time without any cues from the outside world,

finding that would define our biological rhythm as being approximately one day (circadian), and not precisely one day.

The second unexpected—and more profound—result was that their reliably repeating cycles of wake and sleep were not precisely twenty-four hours in length, but consistently and undeniably longer than twenty-four hours. Richardson, in his twenties, developed a sleep-wake cycle of between twenty-six and twenty-eight hours in length. That of Kleitman, in his forties, was a little closer to, but still longer than, twenty-four hours. Therefore, when removed from the external influence of daylight, the internally generated

of each man was not exactly twenty-four hours, but a little more than that.

the average duration of a human adult’s endogenous circadian clock runs around twenty-four hours and fifteen minutes in length.

suprachiasmatic (pronounced soo-pra-kai-as-MAT-ik) nucleus.

Wakefulness and sleep are therefore under the control of the circadian rhythm, and not the other way around. That is, your circadian rhythm will march up and down every twenty-four hours irrespective of whether you have slept or not.

Although human beings display an unyielding twenty-four-hour pattern, the respective peak and trough points are strikingly different from one individual to the next.

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 percent of the populace. They

chronotype,

In

biological trait—here, the useful variability in when individuals within a collective tribe go to sleep and wake up—that could enhance the survival safety and thus fitness of a species by this amount. And so she hasn’t.

MELATONIN

In this

Melatonin is the voice of the timing official that says “Runners, on your mark,” and then fires the starting pistol that triggers the race. That timing official (melatonin) governs when the race (sleep) begins, but does not participate in the race. In this analogy, the sprinters themselves are other brain regions and processes that actively generate sleep.

For every day you are in a different time zone, your suprachiasmatic nucleus can only readjust by about one hour. It

You may have noticed that it feels harder to acclimate to a new time zone when traveling eastward than when flying westward. There are at least two reasons for this. First,

destruction of brain cells caused by the biological stress of time-zone travel.

adenosine

One consequence of increasing adenosine in the brain is an increasing desire to sleep. This is known as sleep pressure, and it is the second force that will determine when you feel sleepy, and thus should go to bed.

your ears to shut out a sound. By hijacking and occupying these receptors, caffeine blocks the sleepiness signal normally communicated to the brain by adenosine.

The upshot: caffeine tricks you into feeling alert and awake, despite the high levels of adenosine that would otherwise seduce you into sleep.

dark chocolate and

is a common culprit that keeps people from falling asleep easily and sleeping soundly thereafter, typically masquerading as insomnia, an actual medical condition. Also

some people have a more efficient version of the enzyme that degrades caffeine, allowing the liver to rapidly clear it from the bloodstream. Others, however,

slower-acting version of the enzyme. It takes far longer for their system to eliminate the same amount of caffeine. As

For the entire time that caffeine is in your system, the sleepiness chemical it blocks (adenosine) nevertheless continues to build up. Your brain is not aware of this rising tide of sleep-encouraging adenosine, however, because the wall of caffeine you’ve created is holding it back from your perception. But once your liver dismantles that barricade of caffeine, you feel a vicious backlash: you are hit with the sleepiness you had experienced two or three hours ago before you drank that cup of coffee plus all the extra adenosine that has accumulated in the hours in between, impatiently waiting for caffeine to leave. When

IN STEP,

and corresponding sleep pressure continue to increase?

AM

But while your conscious mapping of time is lost during sleep, at a non-conscious level, time continues to be cataloged by the brain with incredible precision.

One last temporal distortion deserves mention here—that of time dilation in dreams, beyond sleep itself. Time isn’t quite time within dreams. It

new bout of sleep and its varied stages each night so as to auto-update our memory networks based on the events of the prior day. This account is one reason (of many, I suspect) explaining the cycling nature of NREM and REM sleep, and the imbalance of their distribution across the night.

HOW

sleep spindles,

The steady, slow, synchronous waves that sweep across the brain during deep sleep open up communication possibilities between distant regions of the brain, allowing them to collaboratively send and receive their different repositories of stored experience. In this regard,

But you’re not awake. Rather, you are sound asleep. So what information is being processed, since it is certainly not information from the outside world at that time?

When it comes to information processing, think of the wake state principally as reception (experiencing and constantly learning the world around you), NREM sleep as reflection (storing and strengthening those raw ingredients of new facts and skills), and REM sleep as integration (interconnecting these raw ingredients with each other, with all past experiences, and, in doing so, building an ever more accurate model of how the world works, including innovative insights and problem-solving abilities).

Why did evolution decide to outlaw muscle activity during REM sleep? Because by eliminating muscle activity you are prevented from acting out your dream experience.

The brain paralyzes the body so the mind can dream safely.

very different theory: sleep was the first state of life on this planet, and it was from sleep that wakefulness emerged. It

Yet sleep violates this reliable pattern. Squirrels and degus are part of the same family group (rodents), yet they could not be more dissimilar in sleep need.

demands more sleep is the need to service an increasingly complex nervous system.

Partially aquatic mammals, they split their time between land and sea. When on land, they have both NREM sleep and REM sleep, just like humans and all other terrestrial mammals and birds. But when they enter the ocean, they stop having REM sleep almost entirely. Seals

After all, absence of evidence is not evidence of absence.

the brain opts to heap much more deep NREM sleep onto its plate. In the battle of importance, NREM sleep therefore wins. Or does it?

We try to recover one (NREM) a little sooner than the other (REM), but the brain will attempt to recoup both, trying to salvage some of the losses incurred.

That humans (and all other species) can never “sleep back” that which we have previously lost is one of the most important take-homes of this book, the