Why We Sleep: The New Science of Sleep and Dreams

by Matthew Walker

First, after waking up in the morning, could you fall back asleep at ten or eleven a.m.? If the answer is “yes,” you are likely not getting sufficient sleep quantity and/or quality. Second, can you function optimally without caffeine before noon? If the answer is “no,” then you are most likely self-medicating your state of chronic sleep deprivation.

When you don’t get enough sleep, one consequence among many is that adenosine concentrations remain too high.

If you didn’t set an alarm clock, would you sleep past that time? (If so, you need more sleep than you are giving yourself.) Do you find yourself at your computer screen reading and then rereading (and perhaps rereading again) the same sentence? (This is often a sign of a fatigued, under-slept brain.)

Of course, even if you are giving yourself plenty of time to get a full night of shut-eye, next-day fatigue and sleepiness can still occur because you are suffering from an undiagnosed sleep disorder, of which there are now more than a hundred. The most common is insomnia, followed by sleep-disordered breathing, or sleep apnea, which includes heavy snoring.

I have provided a link to a questionnaire that has been developed by sleep researchers that will allow you to determine your degree of sleep fulfillment.

Questionnaire

Chapter 3 Defining and Generating Sleep

First is the loss of external awareness—you stop perceiving the outside world.

All these signals still flood into the center of your brain, but it is here, in the sensory convergence zone, where that journey ends while you sleep.

A smooth, oval-shaped object just smaller than a lemon, the thalamus is the sensory gate of the brain. The thalamus decides which sensory signals are allowed through its gate, and which are not.

The second feature that instructs your own, self-determined judgment of sleep is a sense of time distortion experienced in two contradictory ways.

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.

Before bed, you diligently set your alarm for 6:00 a.m. Miraculously, however, you woke up at 5:58 a.m., unassisted, right before the alarm. Your brain, it seems, is still capable of logging time with quite remarkable precision while asleep.

Unlike the phase of sleep where you are not dreaming, wherein you lose all awareness of time, in dreams, you continue to have a sense of time. It’s simply not particularly accurate—more often than not dream time is stretched out and prolonged relative to real time.

That is, memories were being “replayed” at the level of brain-cell activity as the rats snoozed.

During REM sleep, the memories were being replayed far more slowly:

This dramatic deceleration of neural time may be the reason we believe our dream life lasts far longer than our alarm clocks otherwise assert.

the gold-standard scientific verification of sleep requires the recording of signals, using electrodes, arising from three different regions: (1) brainwave activity, (2) eye movement activity, and (3) muscle activity.

Aserinsky also observed that these two phases of slumber (sleep with eye movements, sleep with no eye movements) would repeat in a somewhat regular pattern throughout the night, over, and over, and over again.

At that moment Kleitman and Aserinsky realized the profound discovery they had made: humans don’t just sleep, but cycle through two completely different types of sleep.

REM sleep, in which brain activity was almost identical to that when we are awake, was intimately connected to the experience we call dreaming, and is often described as dream sleep.

NREM sleep received further dissection in the years thereafter, being subdivided into four separate stages, unimaginatively named NREM stages 1 to 4 (we sleep researchers are a creative bunch), increasing in their depth.

with “depth” being defined as the increasing difficulty required to w...

The cerebral war between the two is won and lost every ninety minutes,fn2 ruled first by NREM sleep, followed by the comeback of REM sleep.

Figure 8: The Architecture of Sleep

While it is true that we flip-flop back and forth between NREM and REM sleep throughout the night every ninety minutes, the ratio of NREM sleep to REM sleep within each ninety-minute cycle changes dramatically across the night.

But as we transition through into the second half of the night, this seesaw balance shifts, with most of the time dominated by REM sleep, with little, if any, deep NREM sleep.

We have no scientific consensus as to why our sleep (and that of all other mammals and birds) cycles in this repeatable but dramatically asymmetric pattern, though a number of theories exist.

Forced by the known storage capacity imposed by a set number of neurons and connections within their memory structures, our brains must find the “sweet spot” between retention of old information and leaving sufficient room for the new. Balancing this storage equation requires identifying which memories are fresh and salient, and which memories that currently exist are overlapping, redundant, or simply no longer relevant.

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 contrast, the dreaming stage of REM sleep, which prevails later in the night, plays a role in strengthening those connections.

Functions of sleep types

After several more cycles of work, the balance of sculptural need has shifted.

shift toward the goal of strengthening the elements and enhancing features of that which remains (a dominant need for the skills of REM sleep, and little work remaining for NREM sleep).

Since life’s experience is ever changing, demanding that our memory catalog be updated ad infinitum, our autobiographical sculpture of stored experience is never complete. As a result, the brain always requires a 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.

instead of waking up at eight a.m., getting a full eight hours of sleep, you must wake up at six a.m.

Since your brain desires most of its REM sleep in the last part of the night, which is to say the late-morning hours, you may lose 60 to 90 percent of all your REM sleep, even though you are losing 25 percent of your total sleep time. It works both ways. If you wake up at ten a.m., but don’t go to bed until four a.m., then you will lose a significant amount of your normal deep NREM sleep.

What you are hearing is a sleep spindle—a punchy burst of brainwave activity that often festoons the tail end of each individual slow wave.

One of their many functions is to operate like nocturnal soldiers who protect sleep by shielding the brain from external noises. The more powerful and frequent an individual’s sleep spindles, the more resilient they are to external noises that would otherwise awaken the sleeper.

Through an astonishing act of self-organization, many thousands of brain cells have all decided to unite and “sing,” or fire, in time.

By severing perceptual ties with the outside world, not only do we lose our sense of consciousness (explaining why we do not dream in deep NREM sleep, nor do we keep explicit track of time), this also allows the cortex to “relax” into its default mode of functioning.

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.

Each night, the long-range brainwaves of deep sleep will move memory packets (recent experiences) from a short-term storage site, which is fragile, to a more permanent, and thus safer, long-term storage location.

It is not sensations from the outside that are allowed to journey to the cortex during REM sleep. Rather, signals of emotions, motivations, and memories (past and present) are all played out on the big screens of our visual, auditory, and kinesthetic sensory cortices in the brain.

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).

As you pass into NREM sleep, some of that muscle tension disappears, but much remains. Gearing up for the leap into REM sleep, however, an impressive change occurs. Mere seconds before the dreaming phase begins, and for as long as that REM-sleep period lasts, you are completely paralyzed.

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.

Chapter 4 Ape Beds, Dinosaurs, and Napping with Half a Brain

Considering how biologically damaging the state of wakefulness can often be, that is the true evolutionary puzzle here, not sleep. Adopt this perspective, and we can pose a very different theory: sleep was the first state of life on this planet, and it was from sleep that wakefulness emerged.

Storyline

For now, our most accurate estimate of why different species need different sleep amounts involves a complex hybrid of factors, such as dietary type (omnivore, herbivore, carnivore), predator/prey balance within a habitat, the presence and nature of a social network, metabolic rate, and nervous system complexity.

More intriguing than the poverty of REM sleep in this aquatic corner of the mammalian kingdom is the fact that birds and mammals evolved separately.