Sleep has fascinated thinkers and philosophers since ancient times. However, the science of sleep – the analytical measurement of this crucial physical state – is still in its relative infancy. In fact, in terms of modern research, sleep science has been around for only about fifty years (that’s not to say that others, centuries before, hadn’t tried to explain sleep). In this chapter I introduce you to some of the most influential sleep researchers past and present and highlight the contributions they’ve made to our understanding of sleep. Then I delve deeper into the science of sleep – from what happens to the brain and body as we sleep, to the cycles of sleep and the nature of dreaming.
Scientists of Sleep
Around 350bce the Greek philosopher Aristotle recorded his thoughts on the nature of sleep and sleeplessness. He concluded, for example, that “sleep is, in a certain way, an inhibition of function, or, as it were, a tie, imposed on sense-perception, while its loosening or remission constitutes being awake.” By looking at humans and animals, Aristotle realized that when we sleep our acknowledgment of the senses ceases to function and in this way at least sleep is different from wakefulness. However, it wasn’t until almost two millennia later that scientific investigations into the nature of the brain, as well as into the nature of sleep, made discoveries that now influence the way we think about sleep and wakefulness. In 1842, Edward Binns published The Anatomy of Sleep, the content of which is elucidated by its subtitle, “The art of procuring sound and refreshing slumber at will.” Binns mooted that sleep was an active process over which we have some control, rather than a passive one resulting merely as a consequence of tiredness. He believed that human beings could exert influence over sleep by removing all stimulation.
After Richard Caton, a British scientist working in the late 19th century, had attached electrodes to the scalps of animals, establishing that there was electrical activity in the brain, others were able to make great advances in sleep research. In the 1920s the German psychiatrist Hans Berger became the first to reveal that the human brain operated on a number of different electrical frequencies, which he recorded, calling the readings electroencephalograms (EEGs). Crucially for sleepscience, he demonstrated that the brainwaves active in the humanbrain during sleep were different from those associated with wakefulness, although it was several years before anyone believed him.
Around the same time that Berger was making EEGs in Germany,
Professor Nathaniel Kleitman, a Russian-born American psychologist,
regarded by many as the founder of modern sleep research, was conducting
experiments on himself and others to find more evidence for
the nature of sleep. He spent periods of time underground, living in
Mammoth Cave, Kentucky, to establish what happened to the body
when it was forced to exist in perpetual darkness. He found that the
body works on a circadian rhythm – a 24-hour cycle –
more or less constant whatever the light conditions of our environment.
However, Kleitman’s ambitions went beyond wanting to establish
that our body doesn’t need light and dark in order to follow its natural
rhythms. He wanted to challenge what had become the accepted
wisdom that sleep was a single, linear state of rest. He proved instead
that, as in fact Aristotle had believed centuries before him, sleep was
the obverse side of the same coin as wakefulness – that the two were
both mutually exclusive and interdependent; that they complemented
one another. The result was perhaps his most famous publication, a
book called Sleep and Wakefulness, which he published in 1939.
Kleitman, who lectured at the University of Chicago until he was over 100 years old, had two students who helped to put sleep medicine
firmly on the clinical map. The first, Eugene Aserinsky, with Kleitman,
established that REM sleep existed and that it had a connection with
dreaming. However, it was another student, William Dement, who
examined the connection in detail, firmly concluding that dreaming
happens during REM sleep and publishing his findings in 1958.
Back in Europe, Michel Jouvet, a French neurologist and academic,
dug deep into Dement’s discoveries about the links between dreaming
and REM sleep. He went one step further, establishing through a series
of experiments on cats that many muscles of the body go into a state of
paralysis during REM sleep in order that we can’t act out our dreams.
He called REM sleep a “paradoxical” stage of sleep in which the body
goes into a strange, independent state of alertness.
From the 1960s onward, sleep research became more accepted as
a branch of medicine, especially following French neurologist Henri
Gastaut’s identification of sleep apnoea (see pp.171–4). We still have
much to learn, but the work of these scientists makes the job of understanding
sleep and treating its problems that much better informed.
Hypnagogia and hypnopompia
In the moments before sleeping and before waking, you may
experience dream-like hallucinations. Respectively, these periods
in your sleep cycle are called hypnagogia and hypnopompia. When
you think back to your experiences of falling asleep, perhaps you
remember feeling as though you were falling, or you spun out a
conversation you wish you’d had during the day – the conversation
you have in your head may seem at once realistic and imaginary.
These are the sorts of hypnagogic experiences that people report
when woken from the precipice of sleep.
In the morning, the process of reconnecting with the world,
including the sudden influx of sensory information, can cause
feelings of confusion as you separate the associations and random
images of your dreams from the certain sensations of reality. In a
sense hypnopompia is your waking brain’s way to try to force the
logic of the real world (which you’re waking up to) onto the illogic
of your dream world (which you’re leaving behind). The result is
the feeling of disorientation you have as you come out of sleep.
Rhythms of the day and night
Almost every living thing – including plants and animals and every
individual cell in the body – has a 24-hour rhythm that sees it go
through periods of activity and inactivity, fast metabolism and slow
metabolism, growth and maintenance. We don’t know for certain
why this internal 24-hour rhythm has evolved, but presumably it’s
because that’s the daily cycle of the Earth as it turns on its axis while
orbiting the Sun. We do know that humans have a specific bundle of
between forty and eighty thousand brain cells that act as this internal
metronome – it’s call the suprachiasmatic nucleus and is located in the
hypothalamus at the base of the brain.
Rhythms that are attuned to the earthly 24-hour cycle of day and
night are called circadian rhythms (from the Latin circa, meaning
“about”, and dies, meaning “day”). Our sleep–wake cycle isn’t the only
circadian rhythm we have – our oxygen consumption, urine output, muscle strength and, crucially for sleep, body temperature are just
some of the other human functions that operate on a 24-hour clock.
Think about your own performance over the course of a day. Perhaps
you feel more mentally alert in the morning, and more physically able
later. Interestingly, many World Records are set when athletes compete
in the evening, when physical strength peaks.
In order to be classed as a circadian or biological rhythm, a cycle
needs to persist even without external triggers. Nathanial Kleitman proved that the sleep–wake cycle was inbuilt when he spent three
months underground without any natural light (see p.13). However,
it’s important that the rhythms are able to be reset (they are what is
known as “entrainable”) by exposure to external stimuli, such as light
and heat – and this is how we cope with, for example, time zones.
Finally, the rhythms must repeat once every 24 hours and they must
retain their pattern of repetition regardless of the outside temperature.
The biological clock needs to synchronize with day and night.
Anything that helps it get in step is called a zeitgeber and the most
important zeitgeber we have is light. It’s probably for this reason that
the suprachiasmatic nucleus sits over the optic nerve, through which
the retina of the eye transmits the transitions from light to dark and
back again, to the brain. White light is a combination of all the colours
of the spectrum. Scientists have discovered that, when it hits the back
of the eye, the blue light part of the spectrum strongly activates its own
branch of the optic nerve, straight to the suprachiasmatic nucleus. It
bypasses the area of the brain that slowly perceives dawning light, and
triggers the brain to begin dealing with light information, and set up
its rhythms accordingly, before you actually perceive the light itself.
The Brainwave Revolution
Hans Berger established the existence of brainwaves in the 1920s.
He attached electrodes to his subjects’ heads (see p.13) and called the
recordings electroencephalograms (EEGs). He realized that there was
more than one type of brainwave present in the human brain, which led
to his identifying and naming “alpha waves” (also known as Berger’s
waves). Alpha waves are oscillations in the electrical activity of the
brain that vary at a rate of between 8 and 12 cycles per second (known
as Hertz, or Hz), and Berger noticed that this happens when we’re
awake, but resting with our eyes closed. He then immediately went
on to identify “beta waves” – oscillations of between 12 and 30Hz
– which he said occur when we’re actively thinking or concentrating.
Since Berger made his discoveries, there has been a brainwave revolution.
Berger’s revelations were spot on, but they were only the tip of
the iceberg. Below is a description of each brainwave type, from the
fastest to the slowest, as we understand them.
Among the most frantic brainwaves are beta waves – the more intense
our active thought processes, confusion, concentration or stress, the
faster the beta-wave oscillations. Beta waves characterize wakefulness
and are rarely present during sleep.
Far from there being only one type of alpha wave, scientists now
believe that there are in fact at least three types. The first, as Berger
identified, occurs when we’re in a state of calm rest, but not asleep or
even tired. The second occurs during REM sleep, when alpha waves
are emitted from a different part of the brain to those of wakefulness.
No one understands fully yet why alpha waves occur during
REM sleep, although presumably this has something to do with the
fact that REM sleep is usually when we’re dreaming. The third type
of alpha wave is known as the alpha-delta and it occurs when we’re
in non-dreaming sleep when there should be no alpha waves at all –
it’s just that they “intrude” on the delta waves of sleep (see below).
Alpha-delta intrusion is associated generally with sleep disorders, and
one study published in 2011 has suggested that it may be particularly
prevalent in people who suffer from depression (see pp.185–7).
Slow theta waves occur at 4 to 7Hz and indicate a deep state of relaxation,
such as you might experience during meditation. They also occur
as we drift off to sleep, becoming interspersed among the alpha waves
that we experience as we close our eyes and relax. During this brief
period between sleep and wakefulness, you might experience strange
sensations and hallucinogenic-type visions that characterize a state
known as “hypnogogia” (see box, p.21).
The slowest brainwaves that we know about are called delta waves,
and it’s these brainwaves that characterize deep sleep (although very
adept yogis might be able to experience them during meditation, too).
They oscillate at frequencies of between 0 and 4Hz. Interestingly, delta
waves occur most frequently in newborn babies, tailing off in their
prevalence as we grow older, so that some people over the age of 75
have little delta-wave activity in their brains at all. What happens to
the delta brainwaves as we age is still subject to much medical debate
but it’s certainly not true to say that these over-75s don’t experience
any deep sleep at all – they do, it’s just that we don’t quite know how.
Cycles and Stages of Sleep
In order to understand how to improve your sleep quality you need
to have a broad overview of what happens to your brain during sleep.
Sleep is not a one-dimensional state. From the moment sleepiness takes
over, you begin a journey through several cycles and several stages.
All healthy adults live their lives in perpetual cycles of roughly 90
minutes each, even during waking hours. During sleep, however,
these 90-minute cycles are made up of distinct stages, plus REM (or
dreaming) sleep. How many stages there are depends whether we’re
using the old system of sleep classification, or the new one described
by the American Academy of Sleep Medicine (AASM) in 2007.
Until recently, sleep had been divided into five stages, beginning with
drowsiness (Stage One sleep), and moving through light sleep (Stage
Two), and two stages of deep sleep (stages Three and Four), and then
REM sleep. Under the new system, sleep is instead separated into two
major categories – N for non-REM sleep and R for REM sleep. N
sleep is sub-categorized as N1, N2 and N3. N1 and N2 are equivalent
to Stage 1 and Stage 2 in the old system, while N3 combines the
old stages Three and Four, the deepest levels. For simplicity, in other
parts of this book, I’ll refer simply to deep sleep (N3), light sleep (N2),
drowsy sleep (N1) and dreaming sleep (R) – unless I need to use the
specific classifications for clarity.
A full sleep cycle
A complete adult sleep cycle lasts 90 minutes (and sometimes up to
100 minutes) and we go through roughly four or five of these cycles
in a healthy night’s sleep. Sleep starts as N sleep. Measuring the onset
of this is very difficult, because it’s impossible to know at what precise
moment we drop from drowsiness into proper slumber. Even when
subjects have electrodes attached to them and their brainwaves are
measured on an EEG machine, we can’t really tell at what exact point
wakefulness turns into sleep. We do know, though, that during this
time – when we’re crossing the threshold into sleep – we might experience
dream-like hallucinations that appear at once real and fantastical.
This state is known as hypnagogia (a similar state, called hypnopompia,
happens as we wake up; see box, opposite).
All of this is characteristic of N1-type sleep, and once we go through
this we arrive at N2 sleep, in which the alpha brainwaves give way to
theta waves, which may be interspersed by “sleep spindles” and “K
complexes”. Each of these is a special kind of brainwave that heralds
the movement into N2 sleep. Sleep spindles are so called because an
EEG chart shows them as a rapid burst of lines. We’re still not really
sure what their purpose is, but some research indicates that they
improve our ability to learn – the more sleep spindles you experience
during a sleep episode, the more you’re able to take on new information
when you wake up. However, why they should occur at this point,
as you’re entering N2 sleep remains a mystery.
K complexes, on the other hand, are high-voltage bursts of brain
activity (they show as extreme peaks and troughs on an EEG graph).
Researchers think that they help to prevent you waking during this
early part of your sleep cycle by dumbing down your response to noise
or other external stimuli. (Interestingly, K complexes will let pass
through any “essential” noise – such as someone calling your name,
or the sound of your own baby crying – so that you wake up.)
Finally, theta waves become interspersed with delta waves, the
slowest brainwaves of sleep. Have you ever felt disoriented or confused
when someone has woken you from sleep? Perhaps they’ve then told
you that they were trying to wake you for a while? If so, you were far into N3-type sleep. This slow-wave sleep is the deepest sleep we
have, and interestingly it’s the time when most people experience night
terrors or sleepwalking.
After a period in N3 sleep, we complete the 90-minute sleep cycle
by “rising” again to N2 (light) sleep and then entering a period of R
(REM/dreaming) sleep. During R, the body undergoes a temporary
paralysis to prevent us from acting out our dreams. In addition, the
brainwave frequencies become similar to those of wakefulness (alpha
and beta waves), which suggest that the brain is active – perhaps the
strongest indication we have that we dream during R.
As the period of R draws to a close, we experience a momentary
waking before beginning the next 90-minute cycle of the night. Most people don’t even notice that they’ve risen to the surface of sleep before
descending again into a new phase.
How long do we spend in each stage?
Although each sleep cycle lasts roughly 90 minutes, the lengths of time
we spend in each stage of sleep within each cycle are not the same.
Over the course of the night, periods of N3 sleep shorten (our longest
period of deep sleep occurs during the first sleep cycle), while periods
of R sleep lengthen, until our last sleep cycle is made up mostly of N2
(light) and R sleep. Overall, we spend up to five percent of the night in
N1 (drowsy) sleep; up to 50 percent of the night in N2 (light) sleep and
up to 25 percent of the night in N3 (deep) sleep. Around 20 percent of
the night is spent in R sleep.
Perfect cycles, perfect sleep
Healthy sleep follows these patterns more or less to the letter. As long
as nothing upsets them, sleep is restorative and restful. However, so
much in life conspires to send sleep out of kilter. The techniques in this
book aim to put all your sleep stages and cycles back in sync.
The Science of Dreaming
We learned at the end of the last section that we spend around 20
percent of our sleeping life in dreaming (R) sleep. Although we do
dream in other stages of sleep, most of our dreams occur while we
experience REM, so if we’re trying to understand the scientific nature
of dreaming, R sleep seems to be the obvious place to start.
R sleep is triggered by electrical impulses from a distributed
network of neurons located in the brainstem, which sits on top of the
spinal column. Slightly higher is the pons, a small area of the brain
(measuring about 2.5cm/1in) that’s responsible for shutting off the
nerves that feed into the spinal column. This causes the temporary
paralysis we associate with R sleep. Higher still in the brain is an area
called the thalamus. This filters messages to the cerebral cortex, the
learning centre of the brain, where we do all our thinking and sorting.During R sleep our eyes move beneath our eyelids and our breathing
quickens and becomes more shallow and irregular. Our heart rate and
blood pressure increase, and men have erections, while vaginal secretions
increase in women. R sleep must be important to our well-being
because we know that we catch up on it as a matter of priority if we
don’t get enough of it on a particular night. So does this mean that
dreams are essential for our well-being, too?
During R sleep we consolidate information. We know this because
one study showed that participants who were deprived of R sleep after
learning a new skill had impaired ability to perform that new skill
when they woke up. A study conducted at Harvard Medical School
and published in 2008 supports the notion that dreaming is a representation
of the real and relevant in our lives. The sleep researchers
discovered that, far from dreaming about events buried within the
vaults of our childhood memories, we’re more likely instead to dream about events that have happened in the last seven days. Furthermore,
many of the participants in the Harvard study claimed that the events
that triggered their dreams were not those they would have considered
to be significant for their daily lives, despite the fact that the brain had
picked them out as needing attention during sleep. (Interestingly, the
same group indicated that most of their dreams were negative.)
We might conclude, then, that far from being some sort of fragmented,
otherworldly existence, or indeed the “royal road to the
unconscious” as Freud suggested, most of our dreams (at least those
associated with R sleep) are essential to or a by-product of learning
consolidation, and memory. They help to sort and process actual
events, even those we think are unimportant, filing them so that they
become an integral deposit in our memory bank.
Other research suggests explanations along similar lines, but rather
than day-to-day events being the triggers for dreams, our emotions
become the dream-weavers. In this theory, scientists think that we need
to consolidate highly emotive or traumatic events into our memory
bank so that these events no longer feel exceptional or stressful. For
example, if you got stuck in an elevator, you might feel claustrophobic
or frightened. Over the subsequent nights, elevators themselves may
feature in your dreams, but it’s more likely that the emotions you had
when you were stuck are reflected in a different set of dream circumstances.
Perhaps, then, your feelings of claustrophobia instead trigger
a dream about drowning – or another situation in which you feel panic
and that you can’t breathe. During the dream scenario, your mind
processes the emotion and files it away appropriately in your memory
bank. It may take more than one night to stop having claustrophobic
dreams, but eventually they subside. Scientists moot that at this point
you have rationalized the traumatic elevator experience and filed it
away with other events that you’ve already dealt with.
The Genetics of Sleep
Although we can influence most aspects of our lives, from our fitness
to our mood, there’s an element of us that’s genetic – a physiological imprint inherited from our forefathers that we can’t readily alter. Some
of the clients I meet are surprised to learn that aspects of our sleep
cycles and sleep patterns fall into this category. In the last two decades,
sleep research has made considerable advances in understanding how
our genetic legacy influences our sleep. How alert we are during the
morning or evening (known as “morningness” or “eveningness”), how
long we sleep for, the length of time we spend in the stages of sleep,
and the patterns of our brainwaves during both dreaming and other
stages of sleep have all been shown to be subject to genetic influence.
As you’re probably already beginning to grasp, sleep is a complex
behaviour and many aspects of it differ considerably from person to
person. This is true even when we compare people who are very close
in age. Research into identical and non-identical twins has provided us
with important clues as to how many of our sleep patterns are determined
by upbringing, the environment and our lifestyle and how many
are influenced by our genetic make-up.
A quick lesson in genetics
Understanding the human genome is one of the most complex, intricate
and fascinating aspects of human biology. Our genetic information is
stored in 23 pairs of chromosomes. We inherit one chromosome in
each pair from each of our parents. Our chromosomes are made up of
DNA (deoxyribonucleic acid) and our genes are special units of DNA.
All genes have different strains, or variants, and these are called alleles.
For example, the gene relating to eye colour can be subdivided into
alleles for both, for example, blue and brown. If you inherit a blue
allele from your mother, but a brown one from your father, you’ll have
brown eyes, because the allele for brown dominates that for blue. To
have blue eyes, you must inherit blue alleles from both your parents.
The health of your genes is not constant. All sorts of factors may
cause “gene mutation” – when changes in DNA modify your genetic
make-up as your cells subdivide. Sunlight, pollution and exposure to
bacteria are all causes of gene mutation. Sometimes gene mutations
can improve our genes, while at other times they may damage them
and cause disease. Or, they may have no effect on our genes at all.
Genes and your biological clock
The most important genetic discoveries relating to sleep have been
made by “chronobiologists”, scientists who study our biological
rhythms. For example, one family of genes, known as the Period genes
(PER1, PER2 and PER3, found on chromosomes 17, 2 and 1 respectively)
relate to our 24-hour metabolic and rest–activity rhythms.
Scientists have now developed a genetic test for PER2 to identify
whether or not a person’s morningness or eveningness is genetically
inherited. Another gene, called the Circadian Locomotor Output
Cycles Kaput (CLOCK for short), is central to the control of circadian
rhythms, but also regulates our weight, affects our susceptibility to
insomnia and can impact our mood. Both CLOCK and PER3 are
among several genes that regulate the biological clock in all kingdoms
of life – including the plant kingdom. Have you ever wondered what
it is that triggers a flower to open its petals to the sun? CLOCK and
PER3 provide part of the answer. (Interestingly, PER3 is also thought
to protect the amount of deep sleep you amass during the night.)
Genes, sleepiness and wakefulness
It won’t surprise you to learn that there are genes that control your
tendencies to sleepiness and wakefulness, too. Adenosine genes (there
are several variants) are biological molecules central to the energy
transfer that occurs in all cells of the body – and they also promote
sleep. In fact, if you drink a cup of coffee and then find you can’t
sleep, it’s probably because the caffeine has “blocked” the messages
the adenosine wants to send your brain to make you sleepy (see p.67).
Genes and how long you sleep
Between 17 and 40 percent of your sleep duration is accounted for
by your genetic inheritance. One of the most important genes in
this process is PROK1, which controls the onset of your “biological
night” – the window of opportunity during which your biological
clock is telling your body it’s time to sleep. In people who are naturally
long sleepers, the window of opportunity is relatively long; in short
sleepers, predictably, it’s relatively short. PROK1 is not the only gene responsible for the number of hours you spend asleep. Recent
research has identified another gene, called ABCC9, which can dictate
sleep need by plus or minus around 30 minutes. One in five people
are thought to have this gene, which works by detecting energy levels
in the cells of the body and triggers sleep when it senses they are
What do genes mean for you?
The study of the genes of sleep is essential to scientific understanding
of how sleep works. With each new link we make between sleep and
genetics we have the potential to unlock more of the codes of sleep.
For you, though, understanding your tendency to be a lark or an owl,
accepting that you may need a longer or shorter time in bed and understanding
that the environment affects your genes are all important
because they help you to tailor your sleep-improvement strategy in
line with your biological make-up. For example, if you’re naturally a
short sleeper, there’s little point in trying to force yourself into sleeping
longer – you’ll only get frustrated. Instead, you need to tune in to your
natural rhythm and capitalize on your window of sleep opportunity as
well as take steps that improve the quality of your sleep.
A Memory for Sleep
I’ve heard people say that a good night’s sleep improves memory.
Certainly, a good night’s sleep is essential for waking alertness and so
learning, but memory – the cognitive embedding of information and
experiences – has a rather more complex relationship with sleep.
A little bit about memory
Memory is subdivided into two main types – declarative (itself divided
into episodic and semantic memory) and procedural. Declarative
memory is our memory of facts and figures, events and occurrences.
It provides the storehouse for our personal history (all the events that
have happened to us, our episodic memory) and learned data (our
semantic memory). Procedural memory, sometimes called “implicit” memory, on the other hand, is our record of how to do things using
our motor skills – from doing up buttons to riding a bike or driving
a car. The term “implicit” derives from the fact that as we repeat a
task, we learn the movements we need to perform it to the point that
those movements become automatic – we don’t consciously recall the
process of how to perform the task, we just get on with it. Declarative
memory, by contrast, is “explicit”, because it represents knowledge we
have to consciously recall when we need it.
It has taken several decades of sleep research for us even to begin to
understand the relationship between memory and sleep. It’s only really
since the discovery of sleep brainwaves and since we’ve been able to
measure them using electrodes that scientists have made any significant
breakthroughs. The summary of the discoveries so far is that consolidation
of declarative memories appears to occur during slow-wave (deep)
sleep, while the consolidation of procedural memories appears to occur
primarily during dreaming sleep, when the brainwaves operate at higher
frequencies, more akin to the alpha brainwaves of wakefulness.
Declarative memory and your sleep
In order to test the relationship between sleep stages and declarative
memory, scientists attach electrodes to participants’ scalps to measure
the sleeping brain to operate at certain frequencies. Several studies
since the early 1990s have shown that declarative memory is improved
when the brain tips into slow-wave sleep. During slow-wave sleep,
regions of the brain associated with memory and learning, specifically
the hippocampus and the neo-cortex, communicate with one
another. New information that has been temporarily stored in the hippocampus
is transferred to the neo-cortex, where it becomes part of
our long-term learning – it’s consolidated. The process of transferring
information from one place to the other is generally slow – it can take
weeks, months or possibly years for full consolidation to occur. In the
process, perhaps unsurprisingly, some of the information is lost.
However, if you were thinking that this means you can play facts
and figures into your brain as you sleep and expect to wake up with
them permanently embedded, you must think again. I know from experience that the process doesn’t work: many years ago I tried it in
an attempt to learn my Latin vocabulary in time for an examination
– it didn’t help! It appears (we don’t know for sure yet) that new information
must have passed into the hippocampus up to at least an hour
or so before you go to sleep in order for consolidation of that type of
memory to take place during sleep. If we learn something immediately
before sleep onset, or try to learn it during sleep, we tend not to be
able to recall the information when we wake up. Think back over
your own experience of learning – did you ever drift off during a class
or lecture? If you did, it’s likely that you had no memory of the last
things you were taught just before you fell into sleep. This is because
that new information hadn’t made it as far as your hippocampus yet.
Getting good amounts of deep sleep is essential for retaining information
you’ve learned over the course of the day. One experiment gave
participants two lists of words to memorize on separate occasions.
They were asked to recall as many words as possible from the first list
on the same day that they had learned them, before having any sleep.
They were asked to recall the second list after a period of slow-wave
sleep. On average, participants were able to recall five more words
after they’d been able to sleep than they could in the test conducted on
their ability to learn during a single period of wakefulness. In short,
deep sleep appears to be an important aspect of learning consolidation.
Procedural memory and your sleep
During R sleep brainwaves mirror those of wakefulness, appearing as
beta, alpha and theta waves. When the word test described above was
conducted to reveal any changes to learning during R sleep it showed
no difference in the number of words the participants could recall
compared with their recall without having had any R sleep. The same
was not true for procedural memory tasks. A common experiment to
test procedural learning – motor-skill learning – is to teach subjects to
draw mirror images. Scientists have found that we’re much better at
remembering how to draw the mirror image of something if we’ve had
a period of R sleep, than if we’ve had no opportunity to sleep, or have
had the opportunity to enter only other stages of sleep.
Interestingly, procedural memories tend to involve functions that we
learn quickly. This is because they relate to our movement “memory”
pathways, particularly the action-related synapses (synapses are the
gaps between our neurones). During R sleep, the synapses are used
again and again, as if the neurons are re-training during sleep. R sleep,
then, is essential for quickly remembering new motor skills. Think
back to when you learned to ride a bike. Once you could do it, you
could do it – you didn’t have to re-learn the motor skills needed for
bike-riding the next time you tried. Even if you were a bit wobbly,
fundamentally, once you’d learned the skill, you’d learned it for ever.
Of course, all this is to dramatically simplify the relationship between
memory and sleep. The links are strong, and our understanding of
them grows almost weekly. Inevitably, by the time you come to read
this, someone somewhere will have already discovered something new
about the relationship between the two.
Your Body During Sleep
As we’ve already seen, sleep is not a one-dimensional, linear resting
state. The mind is busy as we sleep – and so is the body.
Hormones and chemicals
Body and brain function is controlled by a complex interplay of
nerves and chemicals. Hormones are natural chemicals produced by
the body’s glands. They’re secreted into the blood to give instructions
to our cells. For example, the ovaries and testes secrete the hormones
oestrogen and testosterone respectively, and these affect the way we
grow and function, underpinning many of the physical and behavioural
characteristics that make women and men different.
The body does not secrete hormones the whole time – think of the
menstrual cycle, which is guided by the rise, fall and interplay of a
woman’s hormones over the course of a month. Other hormones (in
both men and women) are dependent on sleep onset or a particular
sleep stage, or are secreted only when it’s dark (see box, p.16). Or, they
might vary along a 24-hour rhythm irrespective of sleep.
For example, the pituitary gland secretes growth hormone (GH)
during the day and during times of stress. However, secretions are at
their highest during sleep, and specifically during deep sleep. This is
why young children who sleep very badly might be small for their age.
The body is brilliant at compensating for insufficient deep sleep (if you
don’t get enough one night, it will try to make up for it as soon as you
next fall asleep), which means that in general we tend to get the right
amounts of GH to remain healthy. GH is also implicated in a healthy
immune system, in mental well-being and in the ageing process.
The sex hormones, on the other hand, have a more complex relationship
with sleep. For a start the secretion of the sex hormones is
not constant over the course of our lives; instead it changes according
to whether we are, for example, going through puberty or (in women)
pregnancy, or whether we’re entering old age. In the majority of
children, secretion of two of the sex hormones, follicle-stimulating
hormone and luteinizing hormone, occurs at the onset of sleep. During
puberty the amount secreted at night increases. With the advent of
adulthood, the body secretes more of these hormones over the course
of the day so that the rate of release is roughly the same both by night
and by day. Testosterone, though, which is present in girls as well as
boys, seems linked to the first episode of dreaming sleep we experience
at night. Lack of sleep appears to dramatically lower levels of testosterone
in the blood of young adults, but the levels return to normal as
soon as the sleep deficit has been overcome.
Sleep and your nervous system
The human nervous system consists of two main parts: the central
nervous system (CNS), which is the brain and nerve tissue in the
spine; and the peripheral nervous system (PNS), which is everything
else and isn’t protected by bone. The brain is the control centre for
both systems. The PNS is itself subdivided in two: the somatic nervous
system (the nerves that control our muscles) and the autonomic nervous
system (ANS), which controls automatic functions such as heartbeat,
breathing rate and salivation. Digestion is governed also partly by the
ANS, as well as by its own set of nerves – the enteric nervous system. As well as our CNS and PNS, we have a set of 12 pairs of cranial
nerves, which emerge from the brain rather than the spinal column.
Of these 12 pairs, the most important for sleep is the vagus (from
the Latin for “wandering”) nerve. Once it leaves the brain, this nerve
wends its way through the body both controlling organs and passing sensory information from the organs to the brain. Overstimulation of
the vagus nerve during wakefulness can make you feel dizzy or may
lead to fainting. If you’ve ever felt wobbly after vomiting or experienced
a head rush when you’ve felt squeamish, those experiences are a
result of overstimulation of your vagus nerve. Prolonged overstimulation,
which can happen if you’re under stress, has a dramatic effect on
sleep – reducing levels of dreaming sleep over the course of a night.
Finally, lack of sleep can itself affect the nervous system. Many
studies have shown that the ANS is severely disrupted in shift workers,
whose biological clocks step out of rhythm with “normal” night and
day. Even if you don’t work shifts, if you find it hard to get to sleep, or
to stay asleep, to the extent that your biological clock starts to tick out
of sync, there may be implications for your ANS, and in particular its
regulation of the rhythms of your heart. Furthermore, we know that
severe lack of sleep may lead to feelings of irritability, wooziness, or
being out of touch or even, in extreme cases, feelings of being disconnected
from the world. This in itself suggests that sleep is essential for
the health of the nervous system – when we don’t get enough of it, we
feel we might be going slightly mad.
What happens to my hormones if I don’t get enough sleep?
Persistent sleep deficit has consequences for four main hormones in
your body, as follows.
1. Growth hormone (GH) deficit: Growth hormone is essential for
your overall health and the body’s repair systems and secretion
usually peaks during deep sleep. Your body is very good at
making up the deficit of the occasional bad night (see p.31), but
prolonged deficit forces the body to secrete the hormone at other
times to try to repay the debt. However scientists think that,
secreted outside deep sleep, the hormone’s efficacy is diminished.
2. High cortisol: Cortisol is a stress hormone. If you have a sleep
deficit, its levels remain high in the evening (when they should
dip), making it harder to fall asleep and creating vicious cycle.
3. Insulin and glucose confusion: During healthy sleep, blood
glucose levels rise, so levels of the hormone insulin rise too in
order to move excess glucose out of the blood and store it as
glycogen in the cells. Lack of sleep impairs the insulin response,
leading to rising blood sugar – and potentially diabetes.
4. Too little leptin: The “satiety” hormone, leptin helps us to feel
full when our calorie intake has reached appropriate levels.
If you have had too little sleep, your leptin secretions may be
up to a third lower than healthy sleepers. As a result you may
consume roughly 900 calories a day more than you actually
need in order to feel full – which can lead to obesity.
Sleep and your heart
The heart and circulation are controlled by the autonomic nervous
system (ANS; see above). The ANS is itself divided up into the parasympathetic
and sympathetic nervous systems. Via the vagus nerve,
the parasympathetic nervous system slows down the heart and the
sympathetic system speeds it up. The heart itself has pacemakers that
provide a basic rhythm of around eighty to 100 beats per minute.
The different sleep stages affect the heart in different ways. During N
sleep the ANS is relatively stable. With the body at complete rest both
physically and mentally, and with no external influences, the heart and
breathing become more settled than at any other time. Unfortunately,
R sleep upsets this perfect state of equilibrium.
During R sleep the whole ANS is not as well regulated as usual. It’s
rather like a faulty thermostat – if the thermostat is set at 20°C (68°F),
when it’s functioning properly it will turn off at 22°C (72°F) and turn on again at 18°C (64°F). If the electronics go awry, it might turn
off at 25°C (77°F) and on again only when it reaches 15°C (59°F).
During R, ANS control of our organs is similar – the trigger points for
maintaining our organs become looser. Rather than ticking over, the
sympathetic and parasympathetic nervous systems experience surges
of activity – a sudden braking because the heart has started to beat too
fast and a sudden acceleration because it has become too slow.
In a healthy individual this is not much of an issue, but if you suffer
from heart or circulatory disease it does become a problem – and is
probably why the rates of heart attack during sleep peak after 4am:
the later stages of sleep contain the longest periods of R.
Sleep and your digestion
The digestive system is governed by the ANS, as well as its own set of
controls, called the enteric nervous system. Although we might think
of digestion as beginning in the stomach, actually it begins in the brain
before moving to the mouth. At the back of the throat lies the oesophagus,
the food pipe that leads to the stomach. The oesophagus is important in sleep terms: at the top of it there’s a striated muscle (one
that is partly involuntarily and partly voluntarily controlled) called
the “cricopharyngeus”. This muscle is unusual because, unlike all the
other striated muscles in the body, it’s not paralyzed during R sleep,
so that we can swallow. However, at night we do swallow less, which
is good, because each swallow causes a momentary sleep interruption.
Further along the digestive system, in the small intestine, nutrients
continue to be absorbed from our day’s food, but in general intestinal
activity slows down during sleep. Most importantly, the peristaltic
waves that usually carry waste into the anal canal run in reverse,
keeping waste back and so minimizing our need for the loo in the night.
(This is the reason why we often need to go first thing in the morning,
when the system reverses again and the night’s waste is pushed on.)
Sleep and your immune system
The precise relationship between sleep and immunity is still unclear.
We know that fevers tend to be worse at the night. As fever is one of
the ways your immune system fights disease, this suggests that sleep provides support for your body’s infection-fighting mechanisms. We
also know that growth hormone triggers repair in the body during
sleep. Furthermore, when we’re ill one of the first immune responses is
to raise the level of sleepiness, and when we do sleep we spend longer
in deep sleep and less time in dreaming sleep than when we’re well.
Scientists hypothesize that sleep simply provides a means to enforce
physical rest so that available energy can divert to support the body’s
fight against disease. Furthermore immunizations provide better protection
if we have a good night’s sleep after the immunization has
taken place. So if you’re going abroad or are in line for the flu vaccine,
do all you can to sleep well on the night after your injection.
Continue Reading – How Much Sleep do you need? You and Your Sleepiness
Continue Reading – The Science of Sleep
Author: Dr. Chris Idzikowski BSc, PhD, CPsychol FBPsS
Dr Chris Idzikowski is currently Director of the Sleep Assessment and Advisory Service.. His previous appointments include Centre Director of the Edinburgh Sleep Centre (Heriot Row), Visiting Professor, University of Surrey, Deputy Head of the Human Psychopharmacology Research Unit at the Robens Institute of Health and Safety, University of Surrey and Head of Clinical Pharmacology at the Janssen Research Foundation. He started researching into sleep more than 20 years ago when he worked at Prof Ian Oswald’s sleep laboratory at Edinburgh University’s Department of Psychiatry before researching into fear and anxiety at the Medical Research (MRC)’s Council APU in Cambridge.
An expert on sleep and its disorders, Dr Idzikowski has served as Chairman of the British Sleep Society, and has sat on the boards of the Sleep Medicine Research Foundation, the European Sleep Research Society and the U.S Sleep Research Society. Formerly Chairman of the Royal Society of Medicine Forum on sleep and its disorders (now the Sleep Medicine Section) , he has held many honorary appointments, both health authority (Oxford) and University (e.g Queen’s University of Belfast).