The Secret World of Sleep: The Surprising Science of the Mind at Rest

The Secret World of Sleep: The Surprising Science of the Mind at Rest

by Penelope A. Lewis

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In recent years neuroscientists have uncovered the countless ways our brain trips us up in day-to-day life, from its propensity toward irrational thought to how our intuitions deceive us. The latest research on sleep, however, points in the opposite direction. Where old wives tales have long advised to "sleep on a problem," today scientists are discovering the truth…  See more details below


In recent years neuroscientists have uncovered the countless ways our brain trips us up in day-to-day life, from its propensity toward irrational thought to how our intuitions deceive us. The latest research on sleep, however, points in the opposite direction. Where old wives tales have long advised to "sleep on a problem," today scientists are discovering the truth behind these folk sayings,and how the busy brain radically improves our minds through sleep and dreams. In The Secret World of Sleep, neuroscientist Penny Lewis explores the latest research intothe nighttime brain to understand the real benefits of sleep. She shows how, while our body rests, the brain practices tasks it learned during the day, replays traumatic events to mollify them, and forges connections between distant concepts. By understanding the roles that the nocturnal brain plays in our waking life, we can improve the relationship between the two, and even boost creativity and become smarter. This is a fascinating exploration of one of the most surprising corners of neuroscience that shows how science may be able to harness the power of sleep to improve learning, health, and more.

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Editorial Reviews

Publishers Weekly
Most of us have some vague impression of the scientific explanations for sleep—resting, reorganizing our thoughts, etc.—but probably no real idea of why or how these things work; luckily Lewis is able to fill in the gaps in her concise and accessible book. As director of Sleep and Memory Lab at the University of Manchester, she is an authority in field and presents her research in an easy-to-read manner. The book starts with the basics: what is sleep? Lewis offers a working "loose definition," is that it's "an inactive time during which an organism responds less than usual when poked or disturbed, but from which it can be roused if danger threatens." From there she explores several possible "reasons" for sleep, including the way the sleeping brain bolsters our ability to remember things (like someone's name, or the way to a friend's house) by something called "memory rehearsal," a reenactment of the information at the "neural level." Lewis also confirms a truth we may have known intuitively, if perhaps had yet to see confirmed by scientific study: "sleep-deprived people are more easily frustrated, intolerant, unforgiving, uncaring, and self-absorbed than they would be if they were properly rested." (Sept.)

There is much to fascinate in this nippy primer on the biology and behaviour associated with snoozing...from the latest on narcolepsy to the sleep-inhibiting qualities of smoked meat, this is wide-awake science

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The Secret World of Sleep

The Surprising Science of the Mind at Rest

By Penelope A. Lewis

St. Martin's Press

Copyright © 2013 Penelope A. Lewis
All rights reserved.
ISBN: 978-1-137-38697-7


why sleep?

Do amoebas sleep? There are certainly times when they ball up and become inactive, but the true answer to this question depends on how you define sleep — and it turns out that there's more than one way to do that.

Using minimalist criteria, sleep can be thought of as an inactive time during which an organism responds less than usual when poked or disturbed, but from which it can be roused if danger threatens. This inactivity seems to have a purpose, since animals that are disturbed during such sleep invariably try to make up for it later on (we call this rebound sleep). Under this loose definition, amoebas actually do sleep. They stop moving, ball up, and become unresponsive even when prodded. They do this for hours at a time, normally at night, and they exhibit rebound if kept on the move and deprived of this restful state. Insects, fish, and amphibians also sleep. In fact, every member of the animal kingdom appears to snooze at one point or another. In the case of wasps and others with antennae, this is particularly obvious since these appendages tend to droop when they snooze, signaling relaxed inattention to the environment.

Is sleep just something animals do when there are no demands on their time? Quite the reverse: Sleep is often a risky business. Most animals live in a predatory environment in which they are extremely vulnerable when they are not alert to surrounding danger. Many creatures could easily end up as a tasty snack if a predator manages to sneak up on them without being detected. This isn't only true for tender little critters like mice, parakeets, and tadpoles. Giraffes, for instance, take about 15 seconds to get to their feet after lying down for a snooze, so they are out of luck if a hungry lion happens to be in the area (it is probably for this reason that giraffes mainly sleep standing up or leaning against a tree, yet they also need to lie down for a short time every night to get some high-quality zzzzs).Perilous as it is to snooze, it seems that sleep is so necessary that, like giraffes, other animals simply have to take the risk. Some, such as the parrot fish, which lives in wide open waters, have evolved clever strategies to limit the danger of catching forty winks. The parrot fish does this by creating a slimy, foul-tasting envelope around itself in order to deter predators who might otherwise have thought it would make a tasty nibble. Bottle-nosed dolphins and many species of birds and ducks have developed "split-brain" sleep, in which the brain activity patterns which indicate it is snoozing are limited to one side of the brain (called a hemisphere) at a time. The other hemisphere stays awake and controls an eye which keeps watch for dangers and also orchestrates some basic movements like swimming or using a flipper to stay afloat. Split brain sleep poses an interesting question regarding consciousness: Is an animal truly awake or aware when only one hemisphere is active?


So what is the purpose of sleep? Surely something which is so widespread across the animal kingdom yet so dangerous and time-consuming must serve an important function. Alan Rechtschaffen, an important player in the history of sleep research, once said, "If sleep does not serve an absolutely vital function then it is the biggest mistake the evolutionary process has ever made." As this statement suggests, scientists are reasonably unified in agreeing that sleep must be important, but ideas about why it is important vary hugely. One popular suggestion is that snoozing is a way to save energy. After all, animals don't normally move around a lot while they are asleep (unless we are talking about dolphins or other split-brain sleepers), so this state of inactivity must surely save some energy. This is a tempting hypothesis. We know, for instance, that many animals hibernate in order to save energy and hibernation shares many superficial characteristics with sleep — but hibernation typically lasts for months at a time, and body temperatures fall much lower during such periods than during sleep (sometimes to just a few degrees above freezing). Many animals actually warm up during hibernation in order to obtain a proper snooze. This means they are investing energy in order to get some sleep — suggesting that such slumber can't exist simply to make energy savings.

Another way to try and work out why sleep is important is by seeing what happens when we don't get enough of it. Sleep deprivation has been studied in great depth — from crude protocols in rats, who have been deprived of sleep to the point of death in many cruel experiments, to more carefully controlled experimentation on humans, whose brains, hormonal profiles, and ability to attend, remember, and make decisions have all been carefully analyzed after more limited periods of deprivation.

In one classic experiment, rats were housed on top of upside-down flowerpots in the middle of a pool of water. The flowerpot platform was above the water but small enough that the rats fell off it (and got wet!) whenever their muscles went limp (Fig. 1). Since one stage of sleep (REM, or rapid eye movement sleep) is characterized by total limpness of bodily muscles, this effectively meant the rats had a nasty wet awakening every time they reached this sleep stage. Rats who were treated in this way soon lost control of their body temperature, lost weight, and developed skin lesions. Within a few weeks they were all dead. This study suggests that sleep is important for temperature regulation and health in general (i.e., lack of deep sleep eventually leads to death), but it has been heavily criticized due to the excessive stress these rats suffered. As you'd imagine, some of the criticism related simply to the cruelty of this procedure, which would have a hard time getting the approval of any contemporary ethics committee, but some of the criticism is also scientific. If you think carefully about the experiment, you'll soon realize that it is difficult to know whether it was really sleep deprivation that led to the untimely demise of these unfortunate creatures, or if their health problems could instead be attributed to the intense stress of the situation. Subsequent experiments in rats have tried to answer this question, but none has completely satisfied the skeptics.

Studies of sleep deprivation in humans have typically involved much less stressful situations in which, though deprived for long periods (sometimes 11 days or more!), participants are otherwise treated very carefully, know they can bow out of the experiment at any time if they really want to, and therefore have little real cause for anxiety. Nevertheless, these types of experiments have shown that sleep deprivation leads to an increase in the stress hormone cortisol, a small drop in body temperature, and compromised immune function. This suggests that, at the physical level, sleep plays a role in the maintenance of body temperature and immune response. Although not negligible, these effects are a far cry from the drastic responses seen in the rat martyrs of the infamous flowerpot experiment.

More striking than the physical effects, however, are the psychological impacts of sleep deprivation. There is no need to tell you that we humans tend to feel pretty lousy if we don't get enough sleep. Perhaps the most extreme example of this comes from the story of Randy Gardner, a 17-year-old who stayed awake for 11 nights in 1965 (a record at the time) as part of a school science project. In the first few days, Gardner had problems focusing and repeating simple tongue twisters. By the fourth day he showed memory loss and had minor hallucinations (e.g., imagining a street light was a person). After a week his speech had become slow and slurred, and by days nine and ten he had more marked cognitive impairments — for instance, when counting back from 100 he stopped at 65, apparently because he couldn't remember what he was doing. He also showed signs of paranoia, and his speech was slow and without intonation. However, Gardner's movement-based skills did not seem to be impaired. He won a game of pinball against a non-sleep-deprived interviewer on the tenth day of his ordeal. Although he had lost about 90 hours of sleep, Gardner only made up about 11 by oversleeping after the experiment, and he showed no evidence of long-term ill effects (though he was not, perhaps, monitored as thoroughly as he would be if such an experiment were to be conducted today, and I would certainly not recommend that any readers try this for themselves).

Gardner's story serves to illustrate many of the impacts of sleep deprivation which have been shown by more carefully controlled and scientific experiments in larger groups. Namely, sleep deprivation can lead to moodiness, hallucinations, paranoia, poor memory, difficulty concentrating, and impaired decision making. These functions are all controlled by the brain, so this pattern suggests that sleep, or lack of it, impacts brain function even more than it impacts the body. This shouldn't come as a surprise, really, given that the brain orchestrates sleep and — far from switching off — moves through a complex and highly structured pattern of activities while you slumber. Let's take a closer look.


We know the brain is active during sleep because scientists have spent a lot of time measuring this activity. The most common way to do this is by sticking little pieces of highly conductive metal onto the scalp. These electrodes can detect tiny electrical signals created by nearby brain cells (Fig. 2a). When you're awake, these signals show diminutive but continuously changing responses which can be used as a window on what's going on in there. For instance, electrodes show responses in visual areas of the brain when you see things, responses in auditory areas when you hear sounds, and so on.

While you're awake, there's plenty going on in your brain, so the overall pattern is of lots of small fast responses, and the electrodes produce what looks like a rapidly oscillating wiggly line (Fig. 2b). This is thought to reflect the fact that many of the different signals in there are going in different directions at the same time, so when they are added up they tend to cancel each other out. Picture the ripples in a small lake where ten speedboats are racing around, all going in different directions, and sometimes narrowly missing each other — these would be much messier than the bow waves produced by a single boat going in one direction. As you get drowsy and start to close your eyes, the electrical signals from your brain slow down and get slightly bigger. This slowing becomes more noticeable as you fall asleep — take away a few speedboats so there is less interference, and imagine that the remaining boats get bigger, so they make slightly larger waves (fig. 2c). We can tell when your sleep gets deeper because new types of electrical activity called sleep spindles soon start to appear on our electrodes. These are little bursts of frenetic activity, often stemming from specific areas in the brain. Imagine now that children have started to jump off the remaining speedboats into your lake, and each time they jump, they thrash around in the water for a few moments before being scooped up again (Fig. 2d). As you go still deeper into sleep the process of removing speedboats progresses, and eventually (when all of those interfering bow waves start to settle down) you start to notice some huge rollers developing, almost as though the Loch Ness Monster is stirring up the depths, heaving itself up and down slowly and regularly at one end of the lake. It isn't just that you didn't notice these rollers before — they weren't there. These slow, high-amplitude waves are characteristic of deep sleep (Fig. 2e). They are a sign that, instead of doing lots of very separate tasks (as when disrupted by the speedboats), many areas of the brain are acting together in a coordinated, but slow, fashion.

All the steps I've described so far fall under the general category of non-REM sleep and can be divided into formal sleep stages. The stage when you initially fall asleep (just losing a few speedboats and slowing things down) is called Stage 1 non-REM. The stage when you lose a few more speedboats and the brain activity looks as though children have started to jump in and thrash around in various parts of the lake is called Stage 2 non-REM. The stage when you start to notice a lot of huge monster-induced rollers is called slow wave sleep — sometimes abbreviated SWS — thus named because of the slow movement of these high-amplitude rollers.

Importantly, the brain doesn't just move through these 4 stages of sleep once in a night. It cycles through them repetitively, with each cycle lasting about 90 minutes. Also, the time spent in REM and slow wave sleep is inversely proportional. You get a lot of slow wave sleep and little REM during the first part of the night, and show the reverse pattern later in the night. This means that if REM occurs at all in the first few 90-minute sleep cycles it is very brief — and the same is true of slow wave sleep during the last couple of cycles, when REM dominates.

All of this can be measured with electrodes on the scalp. And it turns out that, although your pulse and body temperature drop somewhat during the deeper stages of non-REM, there aren't too many other changes so far as your body is concerned. But what about rapid eye movement sleep (often called REM)? This sleep stage typically follows slow wave sleep, and although it is very deep sleep (and is indeed the time when we have our most emotional and bizarre dreams), what happens in the brain at this time is somewhat surprising. Instead of carrying on with the deep rolling waves, the sea monster who was generating them seems to calm down, and we instead see a return of the speedboats — almost as many as were there before you fell asleep. To speak more plainly, during REM sleep the electrical activity in your brain resembles the activity we see during drowsy wakefulness, in which it resembles the ripples in a lake with five or six large speedboats which aren't quite moving at full throttle (Fig. 2f). But this isn't the end of the story. The reason REM is named "rapid eye movement" sleep is because your eyes make rapid darting movements, usually under closed lids, during this phase. The eyes are the only area of the body where movement is possible, since all other skeletal muscles are paralyzed (which is why those poor rats kept falling off their flowerpots whenever they got into REM sleep).

How could this complex dance of sleep stages impact memory, mood, and decision making, and why should sleep deprivation lead to a meltdown in some of these systems? I will address all of these questions in later chapters, but for now let's just take a closer look at how sleep impacts memory.


We know Randy Gardner had trouble with his memory after days of sleep deprivation, so losing sleep clearly has a negative impact upon memory. Related to that, there is a large literature of scientific studies which document cases when memory improves after sleep. The best examples are things like playing the piano or riding a bicycle: movement-based skills which don't require a lot of thought, and which we learn to do without necessarily being able to explain how we do them. These skills improve over a night of sleep, and the change can be dramatic.

A good example comes from a (pianolike) finger-tapping task in which people had to make a particular sequence of button presses as many times as possible in a minute. If you imagine numbering your fingers from pinky to forefinger (Fig. 3), with one button per finger, the sequence people had to press was 4–1–3–2–4. When people practiced this during the day they got faster and faster until they eventually plateaued. Performance didn't change much if they were tested 12 hours later after staying awake all day, but if those 12 hours included a night of sleep, most people were much faster, tapping out up to 20 percent more sequences in a minute than they had done the night before.

Matt Walker, the careful Berkeley scientist who did this experiment, first thought the lack of improvement across the day might be due to the fact that we use our hands (and fingers) to do all kinds of complex tasks; these activities might somehow have interfered with memory of the 4–1–3–2–4 sequence. He checked this by asking people to wear mittens during the day between test sessions. This had no effect whatsoever on the result, so the interference hypothesis went out the window. Instead, it looks as though sleep plays an active role in strengthening this type of memory.


Excerpted from The Secret World of Sleep by Penelope A. Lewis. Copyright © 2013 Penelope A. Lewis. Excerpted by permission of St. Martin's Press.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

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