THE DIFFERENT TYPES OF SLEEP

And yet, scientists still don’t know exactly why we sleep! Incredible as it may seem, despite our increasingly detailed knowledge of the mechanisms that make us fall asleep each night, we know very little for certain about the role that sleep plays (follow the Tool Module link to the left).

In purely operational terms, sleep can be defined as a reversible state of reduced sensory and motor interaction with the environment. Coma (see sidebar) and anaethesia are not immediately reversible; hence, they are states distinct from sleep.

In the 1950s, researchers studying sleep by means of electroencephalography (follow the Tool Module link to the left) found that sleep is far from a passive, unitary phenomenon with the sole purpose of helping the mind and body recover from the day’s wear and tear. On the contrary, the researchers found differing patterns of brain waves that let them differentiate two kinds of sleep: non-REM sleep and REM (or paradoxical) sleep. When we analyze the characteristics of these two states of sleep, and the state of wakefulness, we find important physiological differences among them in many different systems of the body.

-  The electroencephalograph (EEG) traces for brain activity during wakefulness and during REM sleep show a similar pattern: low-amplitude, high-frequency waves. In contrast, non-REM sleep is characterized by waves with a higher amplitude and a lower frequency.

- When you’re awake, your sensations are vivid and originate in the external environment. During REM sleep, when you are dreaming, your sensations also are vivid, but are generated internally. During non-REM sleep, sensations are completely absent or very attenuated.

-  When you’re awake, your motor activity is voluntary and practically constant. During non-REM sleep, it is occasional and involuntary. And during REM sleep, it is non-existent, except for the rapid eye movements after which REM sleep is named (what actually happens is that your brain sends out commands for your muscles to move, but these commands are blocked and not executed, resulting in generalized muscle atonia).

-  Eye movements are thus very frequent both when you are awake and when you are dreaming (i.e., when you are in REM sleep), but rare during non-REM sleep.

-  Thought, which is fairly linear and logical when you are awake, becomes repetitive during non-REM sleep and outright bizarre and illogical during your dream (REM) sleep.

Non-REM sleep, which is divided into four distinct stages, seems to be designed specifically for resting. You muscles are more relaxed, and you move them only infrequently, to adjust your body’s position in bed. As the parasympathetic nervous system becomes predominant during this phase of sleep, your general metabolism slows down: your temperature, energy consumption, heart rate, respiration rate, and kidney function are all reduced.

The slow brain waves recorded on EEGs during non-REM sleep indicate that the brain too seems to be resting. The extensive synchronization of neural activity observed in the brain during non-REM sleep (orchestrated by the thalamus, in a departure from its usual role as a switch) provides further evidence that most of the sensory information from the outside world doesn’t even reach the cortex during this kind of sleep.

It is no surprise that in experiments where people are awakened from non-REM sleep, they can recall only vague thoughts and, on rare occasions, detailed dream scenes. William Dement, an important sleep researcher, has summarized non-REM sleep as an idling brain in a moving body.

Conversely, Dement describes REM sleep as the state of an active, hallucinating brain in a paralyzed body. In the mid-1950s, Dement and his colleagues Eugene Aserinsky and Nathaniel Kleitman began running experiments in which they awakened people from REM sleep and discovered that the vast majority of them reported that they had been dreaming. And these subjects could in fact provide detailed accounts of the events in their dreams, sometimes resembling real life but often with some bizarre features.

People’s behaviour and the physiological changes that their bodies undergo during REM sleep are just as strange. For instance, their EEG traces display high-frequency, low-amplitude waves not unlike those recorded when they are awake. In addition, they display the rapid eye movements characteristic of REM sleep, accompanied by pontogeniculooccipital (PGO) spikes on their EEG traces. And 90 to 95% of the people who are awakened from REM sleep say that they had been dreaming.

During REM sleep, the brain’s oxygen consumption, which reflects its energy consumption, is very high–even higher than when someone is awake and thinking about a complex cognitive problem. Equally remarkable is the almost total loss of muscle tonus during REM sleep, which means that we are literally paralyzed while we are dreaming, except that our respiratory and cardiac muscles maintain “essential services” and our eye muscles remain active, producing the rapid eye movements characterizing this form of sleep. (The small muscles of the middle ear also remain active.)

During REM sleep, the body’s inner temperature is no longer well regulated and tends to shift toward the temperature of its surroundings, just as in reptiles. Because human babies spend tremendous amounts of time in REM sleep, care must be taken to keep the rooms where they sleep at a suitable temperature, so that they don’t suffer from excessive fluctuations in body temperature.

Heart and respiratory rates increase during REM sleep, but in an irregular fashion. And lastly, the penis becomes erect and the clitoris becomes engorged, regardless of whether the dreams in progress have any erotic content. In fact, this phenomenon can be used to determine whether a man’s impotence is of physiological or psychological origin.

The recording device most commonly used to study sleep is the electroencephalograph (EEG) (follow the Tool Module link below). The EEG is used to record the results of all of the activity of the cortical neurons by means of electrodes attached to the scalp at standard locations. Researchers used EEGs first to distinguish the brain-wave patterns characterizing sleeping and wakefulness, and then to distinguish the four separate stages of non-REM sleep (see the illustration below). In humans, the EEG traces during REM sleep and Stage 1 non-REM sleep are very similar, so two other devices are used to distinguish them unambiguously: the electromyograph (EMG), which records the degree of muscle contraction, and the electro-oculograph (EOG), which records the movement of the eyes. The EMG can thus be used to detect the muscle atonia associated with REM sleep, while the EOG records the rapid eye movements for which this form of sleep is named. In some animals, the EEG trace during REM sleep is similar in all respects to the EEG trace during wakefulness. It was this paradox that caused French neurobiologist Michel Jouvet to give this form of sleep its other common name, “paradoxical sleep”, when he discovered it in 1959. The following illustration shows typical EOG, EMG, and EEG traces for the state of wakefulness, for the four stages of non-REM sleep, and for REM sleep. Source: Appleton & Lange

THE SLEEP CYCLES IN A NIGHT

At around the same time every evening, we experience a feeling of fatigue, or cold, or lack of concentration that makes us want to go lie down. If we do go to bed then, we generally fall asleep quickly, in less than 10 minutes. Then we descend through all the stages of non-REM sleep, from Stage 1, which is fairly light, to Stage 4, which is very deep. Next we come back up through the heavier stages of non-REM sleep to the lighter ones. Then suddenly, we go into our first short period of REM sleep, lasting 5 to 10 minutes. This ends the first cycle of that night’s sleep. Depending on the individual, this cycle may last a total of 1½ to 2 hours from the time that sleep begins.

(Adapted from Samara/Sommeil Primutam. Cradess)

A complete night of sleep consists of a series of 4, 5, or sometimes 6 of these cycles. At the end of the period of REM sleep that closes each cycle, there is a moment when waking up is very easy and people very often do so briefly. Generally these brief awakenings last less than 3 minutes, and often all people do with them is change body position (later, they retain no memory of having been awake). Then the next cycle begins.

But if someone is overstimulated, it can take an entire cycle for them to fall asleep again. These periods of being awake are longer and more common after the first two cycles of sleep have been completed. That’s why so many people find themselves awake from about 4:00 AM to 6:00 AM, after which they finally succeed in falling back into a deep sleep.

When you fall asleep again after having been awake during the night, you have to go back down through all the stages of non-REM sleep. (Going from a state of wakefulness directly into a period of REM sleep is characteristic of a sleep disorder called narcolepsy.)

Though all of the sleep cycles in a night last about the same amount of time, their composition changes as the night goes on. In the first third of the night, deep non-REM sleep predominates. In fact, the first two sleep cycles include almost all of the deep non-REM sleep that a person gets in the course of the night. Toward the end of the night, however, a higher proportion of each cycle consists of light non-REM sleep and REM sleep, and the periods of REM sleep can last from 30 to 50 minutes. However, a period of at least 30 minutes of non-REM sleep between periods of REM sleep still seems to be necessary even toward the end of the night.

Over the night as a whole, REM sleep accounts for about 20 to 25% of our sleep time, while stages 3 and 4 of non-REM sleep account for about 15 to 20%, and Stage 1 non-REM sleep for about 5%. Hence the largest proportion of the night (50 to 60%, in young adults) is spent in Stage 2 non-REM sleep. The individual’s age is important, because the characteristics of our sleep cycles change as we get older.

DREAMS

In the psychoanalytic theory of the famous Viennese neurologist Sigmund Freud, dreams are a window into the unconscious, revealing desires and feelings that we have repressed since earliest childhood. In The Interpretation of Dreams, published in 1899, Freud suggested that dreams allow us to fulfil these unrecognized desires, to express sexual and aggressive fantasies that we cannot act out in waking life, or to prepare ourselves to cope with situations that cause us anxiety. According to psychoanalytic theory, the interpretation of our dreams could therefore help us to better understand our conscious psychological lives.

Well Scene, Lascaux Caves, France (circa 17 000 B.C.)

Had our ancestors of the Late Paleolithic period noticed that an erection is a physical manifestation that accompanies REM sleep? One of the many interpretations that have been proposed for this painting in France’s Lascaux Caves that shows a man lying down with an erection, a stick with a bird on top of it, and a wounded bison, is that it shows a dreamer and the concept or content of his dream.

In 1977, Harvard University scientists J. Allan Hobson and Robert McCarley proposed their activation-synthesis model of dreams. This was the first neurobiological model of the origin of dreams that explicitly rejected Freudian psychological interpretations. According to this model, the images that we see in dreams originate in totally random nerve impulses, triggered by the release of acetylcholine by the REM-On cells in the brainstem, and the sleeping brain does exactly the same thing with these impulses as it does with ambiguous visual signals that it receives while we were awake: it attempts to assign them a meaning.

In this model, dreams are thus nothing more than the brain’s desperate attempts to extract coherent images from the confusing signals transmitted by the pons (including PGO spikes ). The result of these efforts each night would then be the strange stories told by our “cinema of the mind”, an amalgam of our concerns of the moment, remembered events, and the emotions associated with them.

Hence this model is not very compatible with the theories of Freud. It sees dreams as being triggered not by psychological phenomena such as the expression of unavowed or repressed desires, but rather by simple biochemical feedback loops between some areas of the brain that are phylogenetically very old and others that are more recent. This theory provoked a general outcry among psychoanalysts, even though it received several experimental confirmations in the years after Hobson and McCarley first proposed it.

In 1983, Francis Crick and Graeme Mitchison suggested that dreams are a way that the brain does “housecleaning” to prevent too great a build-up of the information that it receives over the years. This theory is thus based on the assumption that over the long term, an excess of information could impair the activities by which the cortex stores memories.

According to this theory, every night your brain examines the stimuli that it has received during the day and discards any information that has no meaning for you, and the random activation of the cortical connections by the neurons of the brainstem plays a role in this “unlearning” process. But just how your brain would go about sorting what is meaningful from what is not remains hard to explain.

This theory of “active unlearning” might also explain why we do such a bad job of remembering our dreams: if they consist of non-meaningful information that we are supposed to forget, what purpose would it serve to remember them?

In 1986, Crick and Mitchinson further posited that obsessive ideas are the main kind of information that the brain eliminates by means of dreams. This hypothesis thus bore a substantial resemblance to Freud’s idea that dreams serve to purge the brain of harmful psychic tensions.

Another explanation of the function of dreams (or at least the often strange, emotionally charged dreams associated with REM sleep) is based on the finding that babies spend 8 hours per day in REM sleep when they are newborns, and slightly more than that before they emerge from the womb. It has therefore been hypothesized that REM sleep is related to brain development.

This hypothesis is supported by another observation: the muscle paralysis associated with REM sleep is not so complete in infants as in older children and adults. As a result, during REM sleep, infants who are too young even to smile at their mothers when they are awake often make facial expressions that would correspond to fundamental emotions such as happiness, fear, disgust, and surprise in adults. These expressions may be attributed to genetic programming designed to ensure basic communication among members of our species.

But how does this prior knowledge encoded in the genes get transferred into the organization of the nervous system? The answer isn’t obvious, because we simply don’t have enough genes for them to guide the synaptogenesis and encode all of the brain circuits needed for these basic behaviours. For the construction of these circuits to be completed, they must be activated so that they can then participate in their own building process–what is known as epigenetic development. Some scientists believe that the activating mechanism which enables our genetically inherited wiring to be translated into the infant’s actual nervous system consists of the intense endogenous stimuli that accompany REM sleep.

In Michel Jouvet’s view, this hypothesis that dreams serve as the custodian of the memory of a species account for several phenomena–first of all, his own experiments in which cats whose brains had been surgically altered so that their muscles were no longer paralyzed during REM sleep exhibited all kinds of behaviours during REM sleep that were particular to their species. The hypothesis of epigenetic development could also explain why, for example, a cat who has been raised in an apartment in the city and then is allowed to roam freely in the countryside will always know how to hunt a mouse: because she has practiced doing so every night in her dreams. This hypothesis might also explain why human identical twins who have been raised apart from each other sometimes have similar temperaments. But for this “general rehearsal” explanation to make sense, we also need to explain why adult humans, in whom the neural circuits essential for the survival of the species have been laid down long ago, still spend nearly one-quarter of the night in REM sleep.

Jouvet offered such an explanation in 1991. He proposed that in adults, REM sleep serves both to preserve an individual’s personality and to modify it in response to life experiences, so as to adapt more effectively to the environment. This broader approach, in which dreams help to maintain both the genetic bases of personality and learned behaviours that have proven gratifying, clearly places the function of dreams in an evolutionary perspective. But there is no way to be certain that REM sleep is the only factor responsible for such “reprogramming”.

In fact, dreams may actually serve a number of functions at once. Some more recent and still hotly debated hypotheses thus propose that though dreams have a neurobiological basis, they do still serve certain psychological functions.

SLEEP DISORDERS

You don’t fully realize the importance of sleep until for some reason you stop getting enough. If the reason is a conscious decision to go short on sleep, you can counter the harmful effects of sleep deprivation by changing your priorities. But if you are building up a sleep debt because of an involuntary inability to get the quantity or quality of sleep that you need to see to your daily activities, then you are suffering from insomnia.

According to various sources, the less severe form of insomnia, transitory insomnia, affects between 15 and 25% of all Canadians. The difficulties that people with transitory insomnia experience in falling asleep or remaining asleep may be due to stress, or jet lag, or simply drinking too much coffee. Generally, this problem can be solved through better sleep hygiene. The more severe form of insomnia, chronic insomnia, is less common: it affects about 10% of the Canadian population. It is associated with an imbalance in the neurotransmitters that control the start and the duration of sleep cycles, and is also often associated with psychological disorders such as depression.

The causes of chronic insomnia are highly varied. The first possibilities to consider are external factors such as excessive noise, heat, or cold in the sufferer’s environment. In other cases, however, the causes are internal, sometimes of organic origin, and other times of psychological origin. Possible organic causes of chronic insomnia include known illnesses that cause pain, coughing, or difficulty in breathing.

In yet other cases, chronic insomnia may be caused by pathologies associated specifically with sleep. One example is restless legs syndrome, in which the individual often experiences two different sets of symptoms. The first set are the symptoms of the syndrome itself: very unpleasant sensations of tingling or burning in the legs, accompanied by an irresistible urge to move them. These symptoms usually occur during the evening and at night. They are most likely to arise when a person’s legs are still, and are at least partly relieved if he or she moves them voluntarily. Thus restless legs syndrome can make it harder for people to fall asleep, by forcing them to get up out of bed and walk around.

The second set of symptoms that can contribute to chronic insomnia, and that often accompanies restless legs syndrome, is known as periodic limb movement disorder. It involves involuntary leg movements that occur every 5 to 90 seconds while a person is in deep sleep. These movements affect the muscles of the lower limbs in general, most often causing flexion in the feet and toes, but sometimes also involving the knees and hips. These involuntary movements not only disrupt the sufferer’s sleep but can also be highly disturbing for their bedmates.

Sleep apnea is another sleep pathology, in which people stop breathing for varying periods during the night. The sufferers are generally older men who are overweight. This condition can make them wake up hundreds of times during the night, so that they are very tired during the day.

Sleep apnea is caused by a collapse of the respiratory tract that blocks the passage of air and results in heavy snoring. Such collapse is favoured by the slowed breathing and reduced muscle tone associated with sleep. If excess weight compresses the airways, as it does in obese people, then the pharynx will tend to collapse even more. The sleeper’s blood oxygen level then decreases rapidly, causing a reflexive gasping for air. This pattern can cause the sleeper to wake up hundreds of times in the night without being aware that he is doing so, and he gets up in the morning exhausted. If left untreated, sleep apnea can cause cardiovascular problems and significantly shorten life expectancy. Two ways to improve the situation are to lose weight and to avoid sleeping on your back.

Depression and anxiety are two other conditions that can disrupt sleep considerably. The effect of depression is usually to make people wake up too early in the morning, while anxiety leads both to difficulty in falling asleep and to awakening during the night. Lastly, people’s insomnia may be described as psychophysiological when they have gone through a period of insomnia with a well defined cause and as a result have become negatively conditioned with regard to sleep. They have lost confidence in their ability to sleep, so that the fear of not sleeping keeps them awake all on its own.

Living with insomnia is not restful, to say the least, but neither is the opposite: living with any of the various forms of hypersomnia, of which narcolepsy is the best known. Hypersomnias are not simply a matter of sleeping too much in the daytime because you are not sleeping enough at night. Instead, they are the result of a particular malfunction of the neuronal wakefulness network or the anti-waking system.

The terms parasomnias refers to a whole range of abnormal phenomena that occur during sleep, such as somnambulism (sleepwalking), night terrors, and bruxism (tooth grinding). Parasomnias are especially common in children.