Welcome to Your Brain (34 page)

wake up if they are prodded hard enough. Most animals sleep at night, which makes sense because

it’s hard to see (or be seen) in the dark. Sleep allows animals to conserve energy and to match their

own activity with periods of warmth and light.

Whatever sleep’s function may be, it takes a powerful counteradvantage for a species to forgo

sleep entirely. Of the few animals that never sleep, most are fish that must swim to stay alive, such as

skipjack tuna and some sharks, which get enough oxygen only if water runs through their gills at a high

rate. A similar problem is faced by dolphins, which are air-breathing mammals that have to surface

often; they do this by sleeping with only half of their brain at a time so that they can keep moving.

Other nonsleeping animals include cave-dwelling fish and a few mostly stationary frogs, of which it

would be reasonable to ask the converse question: do they ever really wake up?

In lower vertebrates, sleep consists of a continuous rhythm of low brain activity. In reptiles,

electroencephalographic (EEG) recordings during sleep show a slow rhythm in the form of spiky

events, suggesting that many neurons are active in synchrony. These slow-wave spikes are

reminiscent of slow-wave sleep, the deepest stage of sleep in people.

When birds and mammals arrived on the evolutionary scene, a new type of sleep arose: rapid eye

movement (REM) sleep. At the same time, non-REM sleep began to include intermediate stages in

addition to slow-wave sleep. REM sleep is defined by the eye movements themselves (which you can

see if you watch a sleeping person) and the electrical signature of cortical brain activity. This activity

has a spiky quality that resembles awake activity, which earned REM sleep its other name,

paradoxical sleep, because the brain’s activity during REM sleep is not very sleeplike.

REM sleep is when nearly all dreams occur, especially the vivid ones, in humans and other

mammals. Sleeping dogs, cats, and horses make sounds and fidgeting movements during sleep.

Dreamers are prevented from triggering active movements, though, because commands from the

cerebral cortex to drive movement are blocked by an inhibitory center in the brainstem that is

activated during sleep. Inhibition from the cerebral cortex prevents us from acting out our dreams and

probably accounts for the feeling of paralysis that is often reported during dreams, especially

frightening ones. Experts believe its malfunction to be a likely cause of sleepwalking, and also

suggest that it might be a cause of bedwetting by children. The inhibitory center can be removed

surgically; after such an operation, cats arch their backs and engage in mock combat during REM

sleep, suggesting that fights are a common component of cat dreams.

Whether REM sleep and dreaming have a biological function is hotly debated. One of sleep’s

functions may be to “consolidate” memories. Long-term storage of memory seems to undergo a

conversion of some kind over weeks to months, as our memories of facts, events, and experiences are

gradually transferred from an initial storage place in the hippocampus to the cortex. At the same time,

memories of specific episodes are incorporated into more general knowledge known as semantic

memory, in which people remember facts without knowing how they were learned.

A day’s experiences are almost never the subject of dreams the same night but instead are

incorporated into dreams only after a delay of a few days or longer. Perhaps this is because sleep

helps us process them. When sleep is interrupted, some kinds of memories are slower to consolidate.

The critical part of sleep for consolidating memories has been variously suggested to be slow-wave

sleep or REM sleep; deprivation of either stage has some effect on memory reconsolidation, though

most of the evidence (and research) has focused on REM sleep.

One reason that it has been difficult to study sleep’s connection to memory is that sleep

deprivation damages the brain and body. Sleep deprivation induces a stress response in which the

hormone cortisol is secreted. It takes about four weeks of sleep deprivation to kill a rat, and about

two weeks to kill a fruit fly. The longest bout of known wakefulness for a human is eleven days. This

feat, which is recorded in the
Guinness Book of World Records
, is likely to stand because the book

has closed this category due to the health risks. After a few days of sleep deprivation, humans begin

to hallucinate. At such stressful moments, hormones like cortisol are released, and these stress

hormones are known to impair learning. Sleep deprivation’s negative effect on memory can’t be

explained entirely by stress, though, as sleep deprivation still blocks memory consolidation in

animals after their adrenal glands have been removed to keep them from releasing stress hormones.

Did you know? Wake up, little Susie: Narcolepsy and modafinil

Narcolepsy is a disorder in which sufferers inexplicably fall asleep at all times of the

day. This can happen not only during inactivity, but also at exciting moments. The disorder

has been studied in a colony of narcoleptic dogs living at Stanford University. Playing with

one of these dogs proceeds normally until the dog gets too excited, at which point it falls

asleep. Both human and nonhuman sufferers of narcolepsy lack a particular type of the

neurotransmitter peptide orexin. Orexins act on receptors in the hypothalamus, a command

center for the regulation of sleep, aggression, sexual behavior, and other core activities.

Treatments for narcolepsy have not yet taken advantage of the discovery of orexins.

Instead, most treatments stimulate the nervous system by influencing the action of

monoamines, a large category of neurotransmitters that includes serotonin, dopamine, and

noradrenaline. The drugs used for this purpose include certain anti-depressants and

stimulants such as amphetamine and methamphetamine. The problems associated with these

drugs include side effects such as dizziness or, in the case of amphetamine and

methamphetamine, the potential for addiction. Amphetamine can promote wakefulness at

lower doses than those that lead to motor activation, suggesting that amphetamine’s effects

on waking behavior could potentially be separated from its other effects.

One drug that seems to induce wakefulness without affecting motor activity is modafinil

(sold in the U.S., U.K., and other countries as Provigil), a drug that has become popular for

the treatment of narcolepsy. Modafinil and amphetamine both enhance wakefulness in

normal people and narcoleptics; neither has any effect on wakefulness in mice that are

missing a molecule that transports dopamine out of synaptic spaces. This finding suggests

that wakefulness is tied intimately to the brain’s dopamine signaling system.

One of modafinil’s applications is to enhance wakefulness and reduce risk in long-shift

workers. In a U.S. Air Force study, modafinil was almost as effective as Dexedrine (an

amphetamine) in enhancing performance during forty-hour shifts. The pilots showed

increased alertness, confidence, and performance on simulated flight maneuvers. If

modafinil is really not addictive, it is likely to gain in popularity in both narcoleptics and

people who must work long hours.

Did you know? Why are yawns contagious?

Although we associate yawning with sleepiness and boredom, its function appears to be

to wake us up. Yawns cause a massive expansion of the pharynx and larynx, allowing large

amounts of air to pass into the lungs; oxygen then enters the blood, increasing alertness.

Yawning is found in a wide variety of vertebrates, including all mammals and perhaps even

birds, and can be observed in human fetuses after just twelve weeks of gestation. In

nonhuman primates, yawning is associated with tense situations and potential threats.

Functionally, yawns can be thought of as your body’s attempt to reach a full level of

alertness in situations that require it.

Yawns are contagious, as anyone who has attempted to teach a roomful of bored

students knows. The reason for this contagion is not known, though it might be advantageous

to allow individuals to quickly transmit to one another a need for increased arousal. A

video of yawning also increases the frequency of yawning in chimpanzees and in monkeys.

Yawning is not contagious in nonprimate mammals. However, the ability to recognize a

yawn may be fairly general: dogs yawn in response to stressful situations and are thought to

use yawning to calm others. You can sometimes calm your dog by yawning.

The ability to yawn is buried in the brainstem. Some tetraplegics with tumors in their

pons, which block the transmission of cortical movement commands so that they cannot

open their mouths, can still yawn involuntarily. In these patients, the only place in the brain

that can initiate a yawn is a group of neurons in the midbrain that relays movement

commands from the brain to facial muscles. Some researchers believe that yawns may

begin in these neurons. Yawning can even occur in people in a vegetative coma.

A particular oddity of having yawning mechanisms in a place as tightly packed as the

brainstem is that signals can unexpectedly leak from one region to another. For instance,

one side effect of Prozac is that in some women, yawning can trigger clitoral engorgement

and orgasm, an accidental connection that (for a lucky few) would make boring situations

far more interesting.

Seeing, hearing, or even thinking or reading about a yawn is enough to trigger one’s

own yawning circuitry. You may be attempting to suppress a yawn as you read this, as we

did while writing it. (We don’t take it personally.) Seeing a yawn induces activity in areas

of the cortex that are activated by other visual stimuli and social cues. Although we have

outlined why yawns would be contagious—the advantage of sharing an alert signal—we

don’t know exactly what happens in the brain to spread the contagion.

Why would sleep be important for memory consolidation? One possibility is that changes in the

strength of connections between neurons (synaptic plasticity) are driven by neural activity, which can

occur whether an animal is awake or asleep. If neural activity from a remembered episode were

replayed during sleep, it might facilitate memory consolidation in this way. Indeed, some patterns of

waking neural activity are played back during sleep on a remarkably precise timescale, exact to

thousandths of a second. One activity requiring precise sequencing of neural firing is the production

of sounds, such as speech or birdsong. When a bird sings, specific sets of neurons in the bird’s brain

fire in an order that is linked closely to the sequence of sounds in the song. These neurons are

responsible for generating precisely controlled changes in muscle tension that control the bird’s

sound-producing organ, thereby generating the same song every time. Researchers monitored these

neurons while the birds slept and found that the same patterns were generated during sleep. In a sense,

it appears that birds dream about singing their songs.

Non-REM sleep may also involve the playback of waking experience. As a rat runs through a

maze, so-called place cells in its hippocampus fire in an order corresponding to the sequence of

locations that the rat passes through. When the rat is asleep, the same place cells fire again in the

same order. This replay occurs during slow-wave sleep, when dreaming is very rare in humans. The

replayed snippets are typically a few seconds long, suggesting that rats replay moments in the maze,

not necessarily the whole experience.

Synapses in different brain regions follow different rules for the conditions under which plasticity

can occur, and these differences may relate to the phases of sleep. For instance, in the hippocampus,

where initial spatial and episodic memories are thought to be formed, changes in synaptic strengths

require the theta rhythm, a pattern of about eight neural spikes per second that occurs only in awake

animals during exploratory behaviors such as walking—and in REM sleep. For this reason, scientists

associate memory consolidation with REM sleep.

The idea that sleep is important for reconsolidating and redistributing memories provides an

alternative to Freud’s view that dreams express unconscious desires. This piece of psychoanalytic