THE SCIENCE OF SLEEP
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.
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 acknowledgement 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 sleep science, he demonstrated that the brainwaves active in the human brain 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 – which remains 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.192–201). 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.
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 40,000 and 80,000 brain cells that act as this internal metronome – it’s called the suprachiasmatic nucleus and is located in the hypothalamus at the base of the brain.
BODY TEMPERATURE AND SLEEP
Your average body temperature is 37°C (98.6°F), and although many people think this is constant, actually over the course of the day and night body temperature undergoes a circadian rhythm that sees tiny fluctuations above and below the average of approximately 0.5°C (slightly less than 1°F). In a healthy adult, body temperature is at its highest around 11pm. After this peak, it begins to fall, and this is one of the triggers that we think tell the body that it’s time to sleep. Body temperature reaches its lowest point at around 4am. In the 1990s researchers at Cornell University, New York, conducted an experiment in a carefully controlled environment on 44 healthy adults aged between 19 and 82 years old to try to measure the correlation between temperature and our ability to fall asleep. They found that without any external distractions it took less than 45 minutes for participants to fall asleep once their body temperature had begun to come down. The results suggest that the best time for dropping into slumber is when body temperature is falling at its fastest. For this reason, sleep specialists recommend having a hot bath about 90 minutes before you try to sleep. Then, when you get out of the bath, your body temperature falls rapidly. You should also keep your bedroom relatively cool (see pp.15–5).
MELATONIN AND YOUR BIOLOGICAL CLOCK
Your pineal gland, a pea-sized structure that lies in the middle-front of your brain, is your body’s main source of the hormone melatonin. This hormone – sometimes called the vampire hormone – is secreted when darkness begins to fall. Artificial light prevents the pineal gland from beginning production of melatonin and delays the onset of sleep. Once darkness has fallen, melatonin levels continue to rise, peaking between 3 and 4am. Secretion stops altogether as dawn breaks. Crucially, melatonin does not induce sleep in itself – rather it’s a regulator for your biological clock, making sure you sleep during darkness and wake with the light.
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.14). 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.
LARKS AND OWLS
Like most things in nature, your biological clock is unlikely to be 24-hour perfect – it usually runs slower. If it runs faster, you may wake a little earlier than average and feel bright-eyed and bushy-tailed almost instantly. If that sounds like you, you’re known as a lark. If your biological clock runs slower, you’ll want to be up late into the night, but find getting out of bed in the morning a terrible chore. In which case, you’re an owl. You might find that you need to tailor your working life to suit your natural preferences: larks might find nightshifts hard to cope with, while early-morning shifts would not suit owls. It’s also important to remember that it’s not necessarily when you go to sleep or for how long you sleep you need to change to feel more refreshed – but the quality of your sleep once it’s begun.
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.
Hans Berger established the existence of brainwaves in the 1920s. He attached electrodes to his subjects’ heads (see p.14) 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 8–12 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.
Beta waves
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. Faster than beta are gamma waves … but I’ll leave consideration of those until the next book!
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.209–210).
Theta waves
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.23).
Delta waves
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.
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.
Ninety minutes
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 on 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.
New classifications
Until recently, sleep had been divided into five stages, beginning with drowsiness (Stage One sleep), moving through light sleep (Stage Two), 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 One and Stage Two 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 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.)
HYPNAGOGIC AND HYPNOPOMPIC
In the moments before sleeping and before waking, you may experience dream-like hallucinations. Respectively, these periods in your sleep cycle are called hypnagogic and hypnopompic. 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. Perhaps the hypnopompic state 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.
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 5 per cent of the night in N1 (drowsy) sleep, up to 50 per cent of the night in N2 (light) sleep and up to 25 per cent of the night in N3 (deep) sleep. Around 20 per cent 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.
We learned at the end of the last section that we spend around 20 per cent 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.
CAN I CONTROL MY DREAMS?
I’m not a great believer in dreams as a reflection of our hidden thoughts and desires. I do, however, think that we have the ability to control our dreams. When we become aware of our dreams, we’re said to be lucid dreaming, a term coined by Dutch psychiatrist Frederik van Eeden (1860–1932). During lucid dreams the brain displays greater beta-wave activity, the frantic waves most clearly associated with wakefulness, and it’s at this time that some people can become aware of and then take control of their dreams. Some people, it seems, are naturally able to dream lucidly, taking very little time to become quite adept at it; while others may have to learn techniques, and apply these techniques for several months before they achieve lucidity and control. If you’re interested in being able to control your dreams, on pages 149–153 I’ve set out several methods that have been shown to work.
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 byproduct 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.
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), relates 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.76).
Genes and how long you sleep
Between 17 and 40 per cent 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 relatively low.
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.
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.
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.
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.17). 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.
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.34), 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. 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 a 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.
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.
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 80 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.
How can I prevent nighttime reflux?
During the day your saliva neutralizes acid that escapes into the oesophagus before it reaches your throat. At night you produce less saliva, which means that sometimes stomach acid reaches your mouth to cause the burning sensation we know as heartburn or acid reflux. There are several things you can do:
• Lose weight if you’re overweight
• Always eat your evening meal at least three hours before bedtime
• Sit up straight when you eat
• Raise the head-end of your bed by placing something under it, such as a couple of books, blocks of wood, or bricks (using extra pillows for your head alone is not as effective)
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 paralysed 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.
FEELING FULL AND FEELING SLEEPY
In 1920 one doctor observed that a balloon inserted into the middle of the small intestine and inflated sent the subject to sleep. Although his findings appeared accurate, we now know that it’s not the distention of the small intestine that causes the sleepiness, but the contractions that move the food along. We know this because the sleepy effect doesn’t happen if the intestine is filled with water instead of solid food. It also looks likely that some of the intestinal hormones are soporific. However, there’s still lots to learn about the correlation between feelings of fullness and feelings of sleepiness. In the meantime, if you don’t want to feel sleepy at your desk in the afternoon, have a light lunch.
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.)
The precise relationship between sleep and immunity is still unclear. We know that fevers tend to be worse at 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.