Coma is a state
of lost consciousness in which people respond to their environment in only the
most limited way. Coma can results from various causes that affect the brain’s
neural mechanisms. Some of these causes are reversible, but others are not. A
person in a coma seems to be asleep, but sleep and coma are really two very different
states. During sleep, the brain is very active, but when someone is in a coma,
the brain is far less active than normal and consumes less energy.
What happens if you prevent
people from going into REM sleep? The first experiments to examine this
question were conducted by sleep researcher William Dement. Dement monitored sleeping
subjects, watched for the signs of REM sleep in their EEG
and EMG traces, and woke them up as soon as they entered REM sleep.
They then fell back into what was always non-REM sleep. Dement continued this
pattern with them throughout the night to prevent them from getting any REM sleep.
Surprisingly, after two weeks of this treatment, during which the subjects
got practically no REM sleep at all, few or no negative effects on their behaviour
were observed. But during the nights following the experiments, a “rebound”
in REM sleep was observed. In other words, these individuals got far more REM
sleep than normal, as if their brains needed to make up for the REM sleep time
that they had lost.
Similarly, people who are
taking antidepressants such as monoamine
oxidase inhibitors (MAOIs) get little or no REM sleep, but show
few or no signs of resulting harmful effects, even after taking these drugs for
months or even years.
From these observations, we
can draw at least one conclusion: we know that total sleep deprivation is very
harmful to the organism, so if we can do without REM sleep, then non-REM sleep
must be indispensable for our survival.
We should also bear in mind
that it is hard to design an experiment that will selectively suppress REM sleep
without causing some non-specific effects due to the stress of the instrumentation
method or the multiple actions of the various drugs that are used to suppress
When the brain alternates between
non-REM sleep and REM sleep, it is alternating between one state that is energy-efficient
and another that is energy-intensive. During non-REM
sleep, our cortical neurons are activated synchronously and in a sense are operating
at low speed, so that they consume one-third less glucose and oxygen. In contrast,
in REM sleep, these neurons are extremely active and consume just as much glucose
and oxygen as when we are awake, if not more.
People spend more time in non-REM
sleep following days when they have gotten heavy physical exercise or when the
weather has been very hot. People spend more time in REM sleep following days
when they have been exposed to new or unusual situations that have required them
to learn a lot.
THE DIFFERENT TYPES OF SLEEP
Each of us spends about
one-third of our life asleep. Or, to put it another way, by the time you’re
75, you will have spent 25 years sleeping. Sleep is part of the life of all higher
vertebrates. Suppressing sleep for an extended
period has dramatic effects on an organism’s physiological equilibrium.
In short, sleeping is almost as important as eating or breathing.
yet, scientists still don’t know exactly why we sleep! Incredible as it
may seem, despite our increasingly detailed knowledge of the mechanisms that make
us fall asleep each night, we know very little for certain about the role that
sleep plays (follow the Tool Module link to the left).
purely operational terms, sleep can be defined as a reversible state of reduced
sensory and motor interaction with the environment. Coma (see sidebar) and anaethesia
are not immediately reversible; hence, they are states distinct from sleep.
the 1950s, researchers studying sleep by means of electroencephalography (follow
the Tool Module link to the left) found that sleep is far from a passive, unitary
phenomenon with the sole purpose of helping the mind and body recover from the
day’s wear and tear. On the contrary, the researchers found differing patterns
of brain waves that let them differentiate two kinds of sleep: non-REM
sleep and REM (or paradoxical) sleep. When we analyze the characteristics
of these two states of sleep, and the state of wakefulness, we find important
physiological differences among them in many different systems of the body.
The electroencephalograph (EEG) traces for brain activity during wakefulness
and during REM sleep show a similar pattern: low-amplitude, high-frequency waves.
In contrast, non-REM sleep is characterized by waves with a higher amplitude and
a lower frequency.
- When you’re
awake, your sensations are vivid and originate in the external
environment. During REM sleep, when you are dreaming, your sensations also are
vivid, but are generated internally. During non-REM sleep, sensations are completely
absent or very attenuated.
you’re awake, your motor activity is voluntary
and practically constant. During non-REM sleep, it is occasional and involuntary.
And during REM sleep, it is non-existent, except for the rapid eye movements after
which REM sleep is named (what actually happens is that your brain sends out commands
for your muscles to move, but these commands are blocked and not executed, resulting
in generalized muscle atonia).
movements are thus very frequent both when you are awake and when you
are dreaming (i.e., when you are in REM sleep), but rare during non-REM sleep.
Thought, which is fairly linear and logical when you are awake, becomes
repetitive during non-REM sleep and outright bizarre and illogical during your
dream (REM) sleep.
is divided into four distinct stages, seems to be designed specifically for
resting. You muscles are more relaxed, and you move them only infrequently, to
adjust your body’s position in bed. As the parasympathetic
nervous system becomes predominant during this phase of sleep, your general
metabolism slows down: your temperature, energy consumption, heart rate, respiration
rate, and kidney function are all reduced.
brain waves recorded on EEGs during non-REM sleep indicate that the brain too
seems to be resting. The extensive synchronization of neural activity observed
in the brain during non-REM sleep (orchestrated by the thalamus,
in a departure from its usual role as a switch) provides further evidence
that most of the sensory information from the outside world doesn’t even
reach the cortex during this kind of sleep.
It is no
surprise that in experiments where people are awakened from non-REM sleep, they
can recall only vague thoughts and, on rare occasions, detailed dream scenes.
William Dement, an important sleep researcher, has summarized non-REM sleep as
an idling brain in a moving body.
Dement describes REM sleep as the state of an active, hallucinating
brain in a paralyzed body. In the mid-1950s, Dement and his colleagues Eugene
Aserinsky and Nathaniel Kleitman began running experiments in which they awakened
people from REM sleep and discovered that the vast majority of them reported that
they had been dreaming.
And these subjects could in fact provide detailed accounts of the events in their
dreams, sometimes resembling real life but often with some bizarre features.
behaviour and the physiological changes that their bodies undergo during REM sleep
are just as strange. For instance, their EEG traces display high-frequency, low-amplitude
waves not unlike those recorded when they are awake. In addition, they display
the rapid eye movements characteristic of REM sleep, accompanied by pontogeniculooccipital
(PGO) spikes on their EEG traces. And 90 to 95% of the people who are awakened
from REM sleep say that they had been dreaming.
REM sleep, the brain’s oxygen consumption, which reflects its energy consumption,
is very high–even higher than when someone is awake and thinking about a
complex cognitive problem. Equally remarkable is the almost
total loss of muscle tonus during REM sleep, which means that we are literally
paralyzed while we are dreaming, except that our respiratory and cardiac muscles
maintain “essential services” and our eye muscles remain active, producing
eye movements characterizing this form of sleep. (The small muscles of the
middle ear also remain active.)
During REM sleep,
the body’s inner temperature is no longer well regulated and tends to shift
toward the temperature of its surroundings, just as in reptiles. Because human
babies spend tremendous amounts of time in REM sleep, care must be taken to
keep the rooms where they sleep at a suitable temperature, so that they don’t
suffer from excessive fluctuations in body temperature.
and respiratory rates increase during REM sleep, but in an irregular fashion.
And lastly, the penis becomes erect and the clitoris becomes engorged, regardless
of whether the dreams in progress have any erotic content. In fact, this phenomenon
can be used to determine whether a man’s impotence is of physiological or
The recording device most commonly
used to study sleep is the electroencephalograph (EEG) (follow
the Tool Module link below). The EEG is used to record the
results of all of the activity of the cortical neurons by means
of electrodes attached to the scalp at standard locations. Researchers used EEGs
first to distinguish the brain-wave patterns characterizing sleeping and wakefulness,
and then to distinguish the four separate stages of non-REM sleep (see the illustration
In humans, the EEG traces during REM sleep
and Stage 1 non-REM sleep are very similar, so two other devices are used to distinguish
them unambiguously: the electromyograph (EMG), which records
the degree of muscle
contraction, and the electro-oculograph (EOG), which
records the movement of the eyes. The EMG can thus be used to detect the muscle
atonia associated with REM sleep, while the EOG records the rapid eye movements
for which this form of sleep is named.
In some animals,
the EEG trace during REM sleep is similar in all respects to the EEG trace during
wakefulness. It was this paradox that caused French neurobiologist Michel Jouvet
to give this form of sleep its other common name, “paradoxical sleep”,
when he discovered it in 1959.
The following illustration
shows typical EOG, EMG, and EEG traces for the state of wakefulness, for the four
stages of non-REM sleep, and for REM sleep.
When we are sleep-deprived
one night, we naturally catch up by sleeping more on the following ones. But we
don’t catch up on all the stages of our sleep cycles at the same time. The
first night, we give priority to catching up on deep non-REM sleep. This form
of sleep is of fundamental importance, because we use it mainly to restore our
physical functions–for example, by secreting more growth hormone, synthesizing
more proteins, and intensifying the activity of our immune systems. That’s
why some researchers say that if you want to get through the winter without a
cold, it’s more important to get enough sleep at night than to dress warmly
when you go outside!
The second night after you go short of sleep, you
start catching up on your REM sleep a bit. The amount of REM sleep that you get
in a night seems to be related more to the total length of time you sleep that
night. In other words, the more you sleep, the more REM sleep you get.
At around the same time every
evening, we experience a feeling of fatigue, or cold, or lack of concentration
that makes us want to go lie down. If we do go to bed then, we generally fall
asleep quickly, in less than 10 minutes. Then we descend through all the stages
of non-REM sleep, from
Stage 1, which is fairly light, to Stage 4, which is very deep. Next we come
back up through the heavier stages of non-REM sleep to the lighter ones. Then
suddenly, we go into our first short period of REM sleep, lasting 5 to 10 minutes.
This ends the first cycle of that night’s sleep. Depending on the individual,
this cycle may last a total of 1½ to 2 hours from the time that sleep begins.
(Adapted from Samara/Sommeil Primutam. Cradess)
complete night of sleep consists of a series of 4, 5, or sometimes 6 of these
cycles. At the end of the period of REM sleep that closes each cycle, there is
a moment when waking up is very easy and people very often do so briefly. Generally
brief awakenings last less than 3 minutes, and often all people do with
them is change body position (later, they retain no memory of having been awake).
Then the next cycle begins.
But if someone
is overstimulated, it can take an entire cycle for them to fall asleep again.
These periods of being awake are longer and more common after the first two cycles
of sleep have been completed. That’s why so many people find themselves
awake from about 4:00 AM to 6:00 AM, after which they finally succeed in falling
back into a deep sleep.
you fall asleep again after having been awake during the night, you have to go
back down through all the stages of non-REM sleep. (Going from a state of wakefulness
directly into a period of REM sleep is characteristic of a sleep disorder called
all of the sleep cycles in a night last about the same amount of time, their composition
changes as the night goes on. In the first third of the night, deep non-REM sleep
predominates. In fact, the first two sleep cycles include almost all of the deep
non-REM sleep that a person gets in the course of the night. Toward the end of
the night, however, a higher proportion of each cycle consists of light non-REM
sleep and REM sleep, and the periods of REM sleep can last from 30 to 50 minutes.
However, a period of at least 30 minutes of non-REM sleep between periods of REM
sleep still seems to be necessary even toward the end of the night.
Over the night as a whole, REM sleep accounts for about 20 to 25% of our sleep
time, while stages 3 and 4 of non-REM sleep account for about 15 to 20%, and Stage
1 non-REM sleep for about 5%. Hence the largest proportion of the night (50 to
60%, in young adults) is spent in Stage 2 non-REM sleep. The individual’s
age is important, because the
characteristics of our sleep cycles change as we get older.
Whatever their origin, dreams give
rise to free associations that have led some scientists, such as Harvard psychiatry
professor and dream researcher J. Allan Hobson, to say that our brain is fundamentally
creative. These scientists theorize that the brain’s spontaneous creativity
can express itself during dreams because of the absence of the many somatic, cognitive,
moral, and other constraints that suppress it when we are awake.
artists and scientists have even said that some of their most famous creative
efforts were inspired by dreams: The Devil’s Sonata, by Giuseppe
Tartini; the fable The Two Pigeons, by Jean
de La Fontaine; the discovery of the structure of the benzene molecule,
by Auguste Kekulé; and the discovery of the chemical
transmission of nerve impulses, which earned Otto Loewi
the Nobel Prize in Physiology in 1936.
There is a common belief that you
can learn in your sleep by listening to a recording of the information that you
want to learn. Unfortunately, there is no scientific evidence to support this
claim. Experiments have shown that the few things that the subjects remembered
the next day were things that they had heard during their brief spontaneous
awakenings during the night.
the brain seems to have a lot of trouble in assimilating new information from
the outside world during the night. For example, if you have to get up in the
night for one reason or another, the next morning you will often have no recollection
of having done so.
The discoveries about the effectiveness
protocols for training athletes support the idea that dreams serve
to maintain adaptive behaviours. Of course, when athletes use visualization as
a training tool, they are awake. But the fact remains that some dreams are highly
visual and might therefore affect how well we perform when awake.
"A person's waking
life is a dream guided by the senses."
There is still plenty
of uncertainty about the function or functions of sleep in general (follow the
Tool Module link to the left). But the possible functions of dreams and the mechanisms
that make us have them remain even more mysterious. That is why there are such
a wide variety of theories that attempt to explain dreams and to characterize
between dreams and REM sleep. Some of these theories are mutually compatible,
while others are mutually exclusive. Here is an overview of some of the most controversial
In the psychoanalytic theory of
the famous Viennese neurologist Sigmund Freud, dreams are a window into
the unconscious, revealing desires and feelings that we have repressed since earliest
childhood. In The Interpretation of Dreams, published in 1899, Freud
suggested that dreams allow us to fulfil these unrecognized desires, to express
sexual and aggressive fantasies that we cannot act out in waking life, or to prepare
ourselves to cope with situations that cause us anxiety. According to psychoanalytic
theory, the interpretation of our dreams could therefore help us to better understand
our conscious psychological lives.
Well Scene, Lascaux Caves, France (circa 17 000 B.C.)
of the many interpretations that have been proposed for this painting in France’s
Lascaux Caves that shows a man lying down with an erection, a stick with a bird
on top of it, and a wounded bison, is that it shows a dreamer and the concept
or content of his dream.
Harvard University scientists J. Allan Hobson and Robert
McCarley proposed their activation-synthesis model of
dreams. This was the first neurobiological model of the origin of dreams that
explicitly rejected Freudian psychological interpretations. According to this
model, the images that we see in dreams originate in totally random nerve impulses,
triggered by the release of acetylcholine
by the REM-On
cells in the brainstem, and the sleeping brain does exactly the same thing
with these impulses as it does with
ambiguous visual signals that it receives while we were awake: it attempts
to assign them a meaning.
In this model, dreams are
thus nothing more than the brain’s desperate attempts to extract coherent
images from the confusing signals transmitted by the pons (including PGO
spikes ). The result of these efforts each night would then be the strange
stories told by our “cinema
of the mind”, an amalgam of our concerns of the moment, remembered events,
and the emotions associated with them.
In 1983, Francis Crick and Graeme
Mitchison suggested that dreams are a way that the brain does “housecleaning”
to prevent too great a build-up of the information that it receives over the years.
This theory is thus based on the assumption that over the long term, an excess
of information could impair the activities by which the cortex stores memories.
According to this theory, every night your brain examines the stimuli that
it has received during the day and discards any information that has no meaning
for you, and the random activation of the cortical connections by the neurons
of the brainstem plays a role in this “unlearning” process. But just
how your brain would go about sorting what is meaningful from what is not remains
hard to explain.
This theory of “active unlearning”
might also explain why we do such a bad job of remembering our dreams: if they
consist of non-meaningful information that we are supposed to forget, what purpose
would it serve to remember them?
In 1986, Crick and
Mitchinson further posited that obsessive
ideas are the main kind of information that the brain eliminates by means
of dreams. This hypothesis thus bore a substantial resemblance to Freud’s
idea that dreams serve to purge the brain of harmful psychic tensions.
hypothesis is supported by another observation: the muscle paralysis associated
with REM sleep is not so complete in infants as in older children and adults.
As a result, during REM sleep, infants who are too young even to smile at their
mothers when they are awake often make facial expressions that would correspond
to fundamental emotions such as happiness, fear, disgust, and surprise in adults.
These expressions may be attributed to genetic programming designed to ensure
basic communication among members of our species.
how does this prior knowledge encoded in the genes get transferred into the organization
of the nervous system? The answer isn’t obvious, because we simply don’t
have enough genes for them to guide the synaptogenesis
and encode all of the brain circuits needed for these basic behaviours. For the
construction of these circuits to be completed, they must be activated so that
they can then participate in their own building process–what is known as
development. Some scientists believe that the activating mechanism which enables
our genetically inherited wiring to be translated into the infant’s actual
nervous system consists of the intense endogenous stimuli that accompany REM sleep.
In Michel Jouvet’s view, this hypothesis that
dreams serve as the custodian of the memory of a species account for several phenomena–first
of all, his own experiments in which cats
whose brains had been surgically altered so that their muscles were no longer
paralyzed during REM sleep exhibited all kinds of behaviours during REM sleep
that were particular to their species. The hypothesis of epigenetic development
could also explain why, for example, a cat who has been raised in an apartment
in the city and then is allowed to roam freely in the countryside will always
know how to hunt a mouse: because she has practiced doing so every night in her
dreams. This hypothesis might also explain why human identical twins who have
been raised apart from each other sometimes have similar temperaments. But for
this “general rehearsal” explanation to make sense, we also need to
explain why adult humans, in whom the neural circuits essential for the survival
of the species have been laid down long ago, still spend nearly one-quarter of
the night in REM sleep.
Jouvet offered such an explanation in 1991.
He proposed that in adults, REM sleep serves both to preserve an individual’s
personality and to modify it in response to life experiences, so as to
adapt more effectively to the environment. This broader approach, in which dreams
help to maintain both the genetic bases of personality and learned behaviours
that have proven gratifying, clearly places the function of dreams in an evolutionary
perspective. But there is no way to be certain that REM sleep is the only factor
responsible for such “reprogramming”.
Taking more than 15 minutes to fall
asleep, being awake for periods of more than 15 to 30 minutes during the night,
and sleeping less than 5 hours per night are all symptoms of insomnia.
For someone actually to be diagnosed with insomnia, they must experience these
symptoms more than 3 times per week, and these symptoms must have harmful
consequences during their waking lives.
It is estimated that about 45% of
all adults snore occasionally and 25% do so regularly. Snoring
is more common in men and in people who are overweight and generally gets worse
The noise of snoring can reach a volume of 90 to 100 decibels,
which is the equivalent of a truck driving past nearby. Snoring can therefore
have major social consequences in snorers’ relationships with their spouses
and other people who are close to them.
The noise of snoring is produced
by a vibration of the walls of the pharynx, due to a partial constriction of the
airway. During sleep, the soft palate, the uvula, and the tongue relax a bit.
If certain other factors also are present, such as insufficient muscle tone, swollen
tonsils, an excessively long soft palate, or obstructed or deformed nasal passages,
the passage of air through the respiratory tract is impeded, which causes it to
Though snoring can sometimes disturb the snorer’s own
sleep, it is not dangerous in itself unless it is a symptom of a more serious
problem, sleep apnea.
There are over 300 duly patented inventions to
prevent snoring. Some of them are designed to keep people from sleeping on their
backs (the position in which snoring is often the worst). Others reposition the
lower jaw or force the nasal passages to open wider. A more radical and permanent
treatment is surgery to remove the uvula and part of the soft palate.
You don’t fully
realize the importance of sleep until for some reason you stop getting enough.
If the reason is a conscious decision to go short on sleep, you can counter the
harmful effects of sleep deprivation by changing your priorities. But if you
are building up a sleep debt because of an involuntary inability to get the quantity
or quality of sleep that you need to see to your daily activities, then you are
suffering from insomnia.
According to various sources, the
less severe form of insomnia, transitory insomnia, affects between
15 and 25% of all Canadians. The difficulties that people with transitory insomnia
experience in falling asleep or remaining asleep may be due to stress,
or jet lag, or simply drinking too much coffee.
Generally, this problem can be solved through better sleep
The more severe form of insomnia, chronic
insomnia, is less common: it affects about 10% of the
Canadian population. It is associated with an imbalance in the neurotransmitters
that control the start and the duration of sleep cycles, and is also often associated
with psychological disorders such as depression.
The causes of chronic insomnia are highly varied.
The first possibilities to consider are external factors such
as excessive noise, heat, or cold in the sufferer’s environment. In other
cases, however, the causes are internal, sometimes of organic
origin, and other times of psychological origin. Possible
organic causes of chronic insomnia include known illnesses that cause pain, coughing,
or difficulty in breathing.
In yet other cases, chronic
insomnia may be caused by pathologies associated specifically with sleep. One
example is restless legs syndrome, in which the individual often
experiences two different sets of symptoms. The first set are the symptoms of
the syndrome itself: very unpleasant sensations of tingling or burning in the
legs, accompanied by an irresistible urge to move them. These symptoms usually
occur during the evening and at night. They are most likely to arise when a person’s
legs are still, and are at least partly relieved if he or she moves them voluntarily.
Thus restless legs syndrome can make it harder for people to fall asleep, by forcing
them to get up out of bed and walk around.
set of symptoms that can contribute to chronic insomnia, and that often accompanies
restless legs syndrome, is known as periodic limb movement disorder. It
involves involuntary leg movements that occur every 5 to 90 seconds while a person
is in deep sleep. These movements affect the muscles of the lower limbs in general,
most often causing flexion in the feet and toes, but sometimes also involving
the knees and hips. These involuntary movements not only disrupt the sufferer’s
sleep but can also be highly disturbing for their bedmates.
Sleep apnea is another sleep
pathology, in which people stop breathing for varying periods during the night.
The sufferers are generally older men who are overweight. This condition can make
them wake up hundreds of times during the night, so that they are very tired during
Sleep apnea is caused by a collapse of the respiratory tract
that blocks the passage of air and results in heavy snoring. Such collapse is
favoured by the slowed breathing and reduced muscle tone associated with sleep. If
excess weight compresses the airways, as it does in obese people, then the pharynx
will tend to collapse even more. The sleeper’s blood oxygen level then decreases
rapidly, causing a reflexive gasping for air. This pattern can cause the sleeper
to wake up hundreds of times in the night without being aware that he is doing
so, and he gets up in the morning exhausted. If left untreated, sleep apnea can
cause cardiovascular problems and significantly shorten life expectancy. Two ways
to improve the situation are to lose weight and to avoid sleeping on your back.
and anxiety are two other conditions that can disrupt sleep considerably.
The effect of depression
is usually to make people wake up too early in the morning, while anxiety
leads both to difficulty in falling asleep and to awakening during the night.
Lastly, people’s insomnia may be described as
psychophysiological when they have gone through a period of insomnia
with a well defined cause and as a result have become negatively conditioned with
regard to sleep. They have lost confidence in their ability to sleep, so that
the fear of not sleeping keeps them awake all on its own.
with insomnia is not restful, to say the least, but neither is the opposite: living
with any of the various forms of hypersomnia, of which narcolepsy
is the best known. Hypersomnias are not simply a matter of sleeping too much
in the daytime because you are not sleeping enough at night. Instead, they are
the result of a particular malfunction of the neuronal wakefulness network or
the anti-waking system.