Sleep is a natural state during which the mind and the body rest. During that state, the consciousness is altered, sensory activity is low and muscle activity is blocked by the brain to avoid possible injuries. The process of sleeping is divided into two main phases: REM and non-REM. They are named after their main distinguishing feature. REM stands for ‘rapid eye movement’ which occurs during that phase. It is characterized by the involuntary quick movements of the eyes.
- Stages of Sleep
- What Exactly is REM Sleep
- What Happens in the Brain During that Stage
- Brain Stem and Its Importance
- Is Eye Movement During REM Sleep that Rapid?
- Body Functions During REM Sleep
- What is REM Atonia?
- The Psychological Perspective of REM phase
- REM Sleep Deprivation
- REM Sleep in the Animal Kingdom
- Possible Functions of REM Sleep
Stages of Sleep
When we fall asleep, we enter the non-REM phase first. Then, after a while, the REM phase occurs. Then the cycle starts all over again. Usually, the cycle go through 8 or 9 times a night. Non-REM sleep is subdivided into 3 separate stages. The duration of each of them is up to 15 minutes. All of them have to occur in order to reach REM sleep where dreams occur. Stage 1 is the lightest stage of sleep. Your eyes are closed but the probability of waking up is big. It usually lasts up to 10 minutes. At stage 2, you fall in a little deeper state. This phase involves some alteration in the body functions in order to prepare you for deep sleep. Your body temperature lowers a little and your heartbeat slows down. Stage 3 is the deepest of the non-REM stages. If someone tries to wake a person during that stage of sleep, the attempt will be extremely hard. If you wake up during that phase, there is a high chance of feeling disorientation and dizziness for a couple of minutes, but this is normal. During that state, the body heals itself. Tissues regrow, bone and muscle are being built. The immune system strengthens. Sleeping changes with aging. When you get older, you enter more rarely in that deep sleep state. The span of sleep also alters. However, older people need as much sleep as young adults.
REM sleep takes place about 90 minutes after you fall asleep. The first REM episode is around 10 minutes. With each cycle, REM sleep gets longer. The last stage of REM may last up to an hour. During that stage, the heartbeat and breathing quicken. Dreaming and other fascinating processes also occur during that stage because the brain is more active compared to the rest of the phases. A baby can be in that deep stage for up to 50% of the sleep duration while an adult is in it only 20% of the time – a peculiarity that is being explored by scientists. During that stage, some important key functions are being performed. Some of them include balancing of the mood, storing memories, etc. It is not very clear how those functions are performed. However, scientists agree that it is during that stage that they occur. REM sleep occurs when different regions of the brain send signals to the cerebral cortex. It is responsible for cognitive functions linked to thinking, learning and storing and organizing information. At this state, your body is in a temporal state of paralysis. This will prevent possible injuries. However, this paralysis is sometimes disrupted in one way or another and a person may start to move and toss while he or she is dreaming. This can lead to injuries including falling from bed or hitting something.
Moreover, this stage of sleep stimulates regions of the brain that are responsible for our ability to learn. The ability to memorize information suffers incredibly from the lack of REM sleep. Also, people deprived of REM sleep cannot remember what they have been taught before falling asleep. Migraines are also linked to the lack of REM sleep.
Dream mechanisms are also not well understood. Science is not able to explain why some signals have a clear purpose, while others are sent on a random basis. These random signals sent to the cortex seem to form dreams since the cortex tries to interpret and find certain meaning in them.
What Exactly is REM Sleep
Rapid eye movement sleep is a unique state of sleep which occurs several times throughout the night. It is characteristic for the sleep of mammals. The name comes from the fast and random movement of the eyes during that stage. The state is also characterized by low muscle tone and the tendency for vivid dreaming. REM sleep is also called paradoxical sleep and desynchronized sleep depending on the literature you stumble upon. The brain stem is active during that stage. The electrical and chemical processes that occur during that phase of sleep include neurotransmitter acetylcholine, while histamine, serotonin, and norepinephrine are absent. The cortical and thalamic neurons are more active during that stage.
REM sleep occurs right after the ponto-geniculo-occipital waves take place. This, in simple words, means that there is electrical activity in the brain stem during the shift between deep and paradoxical sleep. The activity in question is linked to the movement of the eyes. It is interesting that the brain is a lot more active during that phase of sleep compared to the awake state. Studies have shown that the amount of glucose and oxygen used by the brain during the REM sleep is higher. It is up to 40% higher compared to non-REM phases.
What Happens in the Brain During that Stage
The neurotransmitter acetylcholine causes the fastening of the brainwaves and it is used in the processes of waking up and REM sleep. Other chemicals are completely absent during those stages and reappear once it is over. Such chemicals include histamine and serotonin. An experiment showed that injecting acetylcholinesterase inhibitor helps mammals to shift from deep sleep into REM sleep. The experiment was also done with carbachol which acts as a substitute of acetylcholine and it had a similar effect. Orexin and gamma-aminobutyric acid are also linked to REM sleep. Scientists discovered that the chemicals in question are found only during that phase of the sleep cycle and they completely diminish during the deep sleep stage. Changes in the electrical patterns are swift and periodical and require certain time in order for them to happen.
Brain Stem and Its Importance
It is the brain stem that is responsible for the neural activity during the rapid eye movement stage of sleep. Robert McCarley and Allan Hobson proposed a hypothesis about the mechanisms by which this activity plays in dreaming. This hypothesis is called ‘activation-synthesis hypothesis’. It states that REM sleep is controlled by pathways of neurons that turn ‘off’ and ‘on’ during the REM stage. The neurons that trigger REM sleep require acetylcholine to activate. The neurons that participate at the end of that stage activate serotonin and noradrenaline production. The main function of the second neurons is to suppress the REM activating neurons. The REM-on neurons stimulate the neurons that suppress them. This serves as a tool for the shift between the two phases of sleep.
A research which was conducted in the 1990s confirmed the importance of the brain stem in the REM sleep stage. Also, it found out that the limbic and paralimbic systems found in the forebrain which are in charge of emotions were more active than other regions found within the brain. It was also found was that the REM and non-REM stage make opposite part of the brain active during each of the stages.
Is Eye Movement During REM Sleep that Rapid?
It actually is. The movement of the eyes during that stage of sleep is faster than the eye movement during wake state. The duration of the movement is also longer. The average person involuntarily moves his or her eyes six times per minute during REM sleep. During non-REM sleep, the eyes can move separately, while in REM sleep the eyes can only move simultaneously and in coordination. This coordinated movement is linked and follows the ponto-geniculo-occipital waves that occur in the brain stem. That eye movement can also be triggered by a particular dream. The dreamer tends to orient his eyes toward particular action that is happening in the dream. However, there is no clear evidence that proves the relation between dream state and the eye movement. Scientists struggle to explain the phenomenon of blind people who have no clear visual imagery but still move their eyes during the paradoxical sleep. Other scientists think that the rapid eye movement has a purpose. The theory states that eye movement is linked to the memory processing of the visual content stored in the brain.
Body Functions During REM Sleep
Body functions change during REM sleep. Breathing and heart-related processes involving heartbeat and arterial pressure become irregular during that phase. Respiratory reflexes also decrease. As a whole, the brain regions controlling breathing become less active during REM sleep compared to other phases of sleep or at wake state. The heart rate and the arterial pressure change with every oscillation in the brain waves or the movement of the eyes. Sudden movements while sleeping or change of breathing also affect the heart rate and blood pressure.
Penis erection occurs in REM sleep. It is also called nocturnal penile tumescence. If there is an evident erectile dysfunction during the wake state and erection still occurs during that phase of sleep, it becomes clear that the reason for that dysfunction has a psychological root. There is no physiological problem – the patient just needs to get some psychological help to deal with the problem. Females go through an erection of the clitoris during REM sleep as well. It is called nocturnal clitoral tumescence. During that process, the clitoris enlarges and blood flow in the vaginal area increases. That leads to lubrication. The total duration of the erection during that stage of sleep can continue up to three and a half hours a night.
Body temperature is left unregulated during that phase. Mammals become extremely sensitive to the temperature of their surroundings. Some of the mechanisms that small mammals develop in order to preserve their temperature include shivering and faster breathing. However, those mechanisms occur during non-REM phase because during REM sleep, the loss of muscle tone prevents the movement of the body. Neurons that are triggered when the body feels cold are not active during that phase of sleep. REM sleep prevents normal neural thermoregulation, thus it is extremely important to make sure your bedroom is warm enough. Temperature influences the duration of REM sleep. In the phase of deep sleep, the organism needs to have a certain temperature. It is not able to get into the stage of REM sleep if this condition is not met. If the temperature is not in the desired range, the mammal is not able to continue its sleep cycle. This mechanism is developed to protect organisms from hyperthermia. However, scientists found out that this mechanism can be tricked or skipped by artificially warming the brain.
What is REM Atonia?
This condition is characterized by body paralysis. Muscles are not able to move. This is due to the blockage of motor neurons. When the body reaches that stage of sleep, some neuronal changes occur. The neurons decrease the potential of their membrane. By doing so, the brain makes it harder for various stimuli to excite the neurons, preventing triggering of movement. This process is called hyperpolarization and naturally protects the body from injuries and lowers the chance of potential wake up while the body goes through the REM stage. The inability of the muscles to move can be caused by the lack of monoamine neurotransmitters and by some mechanisms used by the body in order to reach that muscle state. The medulla oblongata plays important role in that process. It is located between pons and spine. It is thought that it can activate the paralysis of the body during that phase. However, twitching and reflexive movement can be observed during that state.
There is a disorder that can prevent the body from undergoing this natural state. It is called REM behavior disorder and can make a person move in his or her sleep. Scientists still argue which occurs first: the dream or the act. Some of them believe that the dream makes the patient move according to what happens in the dream. Others believe that muscle movement occurs first, thus shaping certain visual images in the head of the sleeping patient. Some people still may act in their dreaming, without suffering from sleep condition. This comes from suppressed muscle commands.
Other sleep disorders are also connected to muscle atonia. Sleepwalking occurs during slow-wave sleep. Typically, sleepwalkers have no memory of what had occurred.
Narcolepsy has features different from REM behavior disorder. People who suffer from narcolepsy fall in the state of REM atonia too often and when it is unnecessary.
Cataplexy and excessive sleepiness while the patient is in an awake state are typical for that condition. The patient may experience body paralysis after suddenlyfalling asleep, which may occur anywhere and at any time. Hypnagogic hallucinations occur during slow-wave sleep and sleep paralysis when the person starts to wake are also characteristic features of narcolepsy. Psychiatric disorders are also linked to problems with the REM phase.
The Psychological Perspective of REM phase
Dreaming and REM sleep are interrelated. There are many theories and experiments that aim to explore and explain that connection. After its discovery, rapid eye movement sleep has provided a basis for dream investigation. Dream reports are collected by waking people up who are in teh REM phase of sleep. It is a common experimental method. Almost everyone who is awakened up during that state can clearly remember his or her dreams. That information is compiled and used for better understanding of the dreaming process. Another peculiarity of that experiment is that people can provide more vivid descriptions of their dreams. Also, all participants claim that the average length of their dreams are longer compared to those who are asked to determine the duration of their dream after waking up naturally. Lucid dreams are more common in REM sleep. However, some scientists believe that lucid dreaming is a combination of wake consciousness and elements of REM sleep. Dreaming during the rapid eye movement sleep is more structured compared to dreaming that occurs in other phases of sleep. For example, there is clear narration, events in the dream follow a certain structure, the feeling that dreamer has is as if the things in the dream actually happened in real life.
Hobson and McCarley thought that ponto-geniculo-occipital waves participate in the amplification of hallucinatory elements that are common in dreaming. These waves do so by providing electrical excitement to the visual cortex and forebrain. Still, that statement does not have any clear evidence and it is not well accepted. Other explanations for unrealistic and bizarre dreams can be found in the mechanisms that occur during the rapid eye movement sleep. The decreased sensitivity to stimuli enables the brain to explore deeper parts of the subconscious.
Dreaming is also possible during other phases of sleep. People who tend to sleep lightly experience dreaming during the second stage of non-REM sleep. Deep sleepers report that they experience something that reminds them more of thinking and not dreaming during the non-REM stage. Some scientists aim to find the exact connection between dreaming and REM sleep phase. It is evident that there are some gaps that need to be filled in for a distinct connection to be established. This is not only due to the fact that dreaming can occur during the non-REM phases. It also arises from the fact that the well-explored neurological aspects of REM sleep has been proven not to trigger dreaming. Thus, dreaming itself needs to be further explored before linking it to a certain phase of sleep. The opinions of scientists are varied. Some want to separate dreams from REM sleep, while others consider this as an unthinkable act, and both parties have evidence that supports their views.
Researchers have shown that REM sleep boosts creativity. After waking up from REM sleep, the participants showed better result in solving problems which require creative solutions. Sleep boosts creativity by helping the mind to find new combinations of certain associative elements. This process occurs during rapid eye movement sleep. This is not connected to memory, but rather to changes in neuromodulation during REM sleep. Scientists think that cholinergic and noradrenergic neuromodulation is responsible for the creativity boost. There are complex processes that are performed only during that phase. Certain areas of the brains are suppressed while other are encouraged. This allows neocortical reorganization of established structures of information and enables new interpretation to be found. This is impossible during wake state. Previous semantic representations are easily ignored during REM sleep. This enables new paths to be established between various concepts, something which is hardly impossible to achieve when the person is conscious. Thus, creativity increases while we fall into eye rapid movement sleep.
The normal sleep cycle of every organism shifts between deep sleep and paradoxical sleep. In the first phase, the brain waves are slow, large and synchronized. The latter phase is characterized by faster and desynchronized brain waves. The natural cycle of sleeping and awake states is called circadian rhythm. It controls sleepiness and other physiological factors. Sleep can be distributed in various ways according to the daily routine of the organism. Sleep can be obtained in one part of the rhythm or can be distributed throughout the day. Sleep is distributed according to the activities of the animals (whether they are nocturnal or diurnal). Organisms return to regulation of their temperature once the REM sleep is over.
REM sleep occurs four or five times per night in humans. At the beginning of the sleep cycle, those phases are quite short but tend to get longer at the end. After the REM phase is over, many animals and some people wake up or fall into a state of light sleep. This happens for a very short time. It is interesting that the amount of REM sleep per night changes with age. Babies tend to be in REM sleep around 80% throughout their sleep cycle. Adults spend 20-25%, and elders tend to spend considerably less time in that phase. During the rapid eye movement stage, neuron activity is quite the same compared to their function in awake state. That gives the origin of the name ‘paradoxical sleep’ – though the person sleeps, their brain activity functions as if they are wide awake.
As we already mentioned, adults spend 20-25% of their sleeping cycle in REM sleep. This means that the overall REM state is up to 120 minutes per night. The first time a person shifts into REM sleep occurs about 70 minutes after he falls asleep. The total cycle of REM and non-REM sleep is around 90 minutes, and with every new cycle, the length of REM sleep increases.
Moreover, REM sleep is subdivided into tonic and phasic. Theta rhythms in the brain are typical for tonic rapid eye movement sleep. The ponto-geniculo-occipital waves are typical for the phasic REM and it is the phase in which rapid eye movement actually occurs. The muscle response to stimuli is really hindered during phasic REM. There is research that showed that during phasic REM sleep, it is harder to arouse from sleep compared to slow-wave sleep.
REM Sleep Deprivation
Deprivation of REM sleep leads to increased attempts of shifting into REM stage while a person is sleeping. While a person is trying to recover from REM sleep deprivation, stage 3 of non-REM sleep and REM sleep occur faster than usual. He also experiences the so-called REM rebound. This means that the time spent in REM sleep is longer, and the depth is also increased. This information really helps to prove that REM sleep is actually necessary from a biological point of view.
Some symptoms related to REM sleep deprivation include primarily psychological changes such as anxiety, mood swings and irritability. Hallucinations and difficulty to concentrate can also occur. Appetite increases because the same regions that control sleep, also control appetite. But REM sleep deprivation can have some benefits. Depression, for example, can be suppressed by this kind of deprivation. However, such therapy may increase aggression and appetite. It is not clear what causes such results, but a possible hypothesis is that it is the noradrenaline which is responsible for this. It is still not clear if the long-term REM sleep deprivation has a negative effect on one’s psychological health and what exactly it may cause. Currently, the only data that is available from various research states that deprivation of REM sleep increases aggressiveness and sexuality. Such research, however, is conducted on test animals, thus the effect on humans is still not clear.
There is suggestion that short-term REM sleep deprivation can have a positive effect on depression, or at least on those types of depression related to the imbalance of some neurotransmitters. Although sleep deprivation leads to annoyance and mood swings in healthy people, it can cause improvement of depressive conditions. However, the effect is temporal because every second person who participated in the research reported that after getting enough sleep, the symptoms of depression regained their severity. This led to the creation of methods to increase the effect. The patient undergoes a planned REM deprivation period lasting for several days in combination with certain medication to increase the duration of REM effect. Some antidepressants suppress REM sleep but its effectiveness decreases after long-term use. Moreover, sleep deprivation has the same effects as antidepressants because they stimulate hippocampal neurogenesis, although it is not very clear if REM sleep is the cause of that stimulation.
Studies in animals and humans about REM sleep deprivation differ greatly. Evidence showed that animals suffer from greater negative effects of REM sleep deprivation than humans do. Some of the animals were sleep deprived for about 70 days. Other possible reason for these results may be the difference between experimental protocols used on the animals and humans subjects. Those used on animals were more uncomfortable and painful compared to those used on humans. Some methods involved placing a laboratory animal on a platform. The platform floats over water and it is so small that, when the animal loses its muscle tone, it falls into the water. Other methods include rude awakening which has tremendous negative effects on the organism and brain waves trigger shaking of the cage when the animal falls into the REM stage.
Studies have shown some interesting facts about REM deprivation and its effect on different mammals. REM deprivation in rats leads to an inability to learn new information but does not influence in any way affect the old information in their memory. One of the experiments conducted showed that rats know how to avoid painful stimuli by feeling the pain once. They learned that this leads to an unpleasant feeling and avoided the source of it. The case with REM sleep deprived rats was different. They did not learn how to avoid the source of painful stimuli although they had felt pain several times. This is not valid for humans although they were REM sleep deprived for a night. However, the common feature between humans and rats is that the attempts to enter REM sleep increase after the lack of REM sleep. REM deprivation also induces REM rebound. Rats and cats have expressed common reaction to REM sleep deprivation. The excitability of the brain increases. In other words, there is an electrical amplification of sensory signals which leads to easier waking since the muscles react to weaker stimuli. There is another study that revealed what happens to the hindbrain after REM sleep deprivation. It showed that its sensory excitability is decreased. This lowered receptiveness to information of the afferent pathway is due to the increased amplification of pathways that the hindbrain is receptive to.
REM Sleep in the Animal Kingdom
Rapid eye movement sleep is found in mammals and birds. All species have different sleep cycle and the length of REM sleep varies. It is known that predators spend more time in REM sleep than their preys do. Size also matters when it comes to that sleep phase – bigger animals tend to go through a longer REM phase than smaller animals. This may be due to the fact that bigger animals possess higher thermal inertia that allows their brain and body to stay in longer periods of thermoregulation deprivation. The whole cycle of non-REM and REM phases is different for different species. The human cycle is around 90 minutes, cats go through it for 22 minutes, while rats successfully start the cycle anew after 12 minutes.
In the womb, mammals spend a great amount of time in REM sleep on a 24-hour basis. Although for a long time it was considered that only mammals and birds go through REM sleep, recent studies have shown that lizards also experience rapid eye movement sleep. This discovery showed that the origin of REM sleep can be really ancient. This suggests that this kind of sleep was experienced by the common ancestor of all amniotes.
Possible Functions of REM Sleep
It is not clear what functions REM sleep has. However, there are several theories of its function.
The Memory Theory
As a whole, sleep is considered to help memory and the processes related to it. Rapid eye movement sleep is thought to keep and maintain certain types of memory, which include spatial, emotional and procedural memory. Experiments showed that REM sleep improves the learning process in rats. Several hours after the REM sleep, new information is processed, stored and strengthened. These experiments also showed that REM sleep deprivation leads to an inability to learn new information. This is valid especially for the complex processes such as processes involving several consecutive actions. Experiments on humans showed that REM sleep improves the learning of new techniques of body movement and problem-solving. Deprivation of REM sleep showed that it negatively influenced the ability to learn new, complex facts such as memorizing of voluminous stories. Moreover, REM sleep inhibits attempts of the conscious mind to suppress certain thoughts which can have a negative effect on a person’s psychological health.
There is a theory called ‘dual-process hypothesis’ that investigates the relationship between sleep and memory. It states that REM and non-REM sleep are involved in different kinds of memories. Tests have proved that theory to be true. They involved memory tasks that started before sleep and were evaluated in the middle of the sleep cycle or started in the middle of the sleep and were evaluated in the morning. Those tests showed that slow-wave sleep which happens in non-REM sleep is vital for factual memory. If that phase of sleep is enhanced in some way, the memory of that person is found to be stronger. The tests also have shown that naps during the daytime, which consist mainly of non-REM sleep, improve the factual memory, but seemed to show no effect on procedural memory. There is another hypothesis that non-REM and REM phases work together to improve the overall memory.
Another theory states that the function of REM sleep is connected to the removal of certain interaction between cell networks in the cerebral cortex. This prevents the process that is connected to the process of forgetting. Thus, memories that are strong are enhanced further and weaker and useless memories that can be regarded as ‘white noise’ simply are forgotten. This is valid for paradoxical sleep or REM sleep. It is linked to the rapid movement of the eyes during that phase and the ponto-geniculo-occipital waves that occur before the eye movement. One explanation provided for that phenomenon states that those waves also play an important role in memory enhancement.
REM Sleep Enhances the Development of the Central Nervous System
The ‘Ontogenetic Hypothesis’ states that REM sleep is an important phase for the development of the brain. The main reason for that is the neural stimulation that occurs during rapid eye movement sleep, which is required by babies to develop mature neural connections and a healthy nervous system. Studies on that topic found that sleep deprivation at the early stage of development leads to behavioral problems and chronic sleep problems. It also leads to lower brain mass and increased death of neuronal cells. Other findings connected to this hypothesis showed that REM sleep may not have any important function when it comes to adults. When the development of the central nervous system is complete, REM sleep loses its importance. REM sleep is seen as an aftereffect of that development stage in babies.
Evolution is Seen as the Cause of REM Sleep
According to a theory, REM sleep originates from the tonic immobility reflex. This reflex is a defense against predators and is characterized by faking death. This total immobilization is the last defense of the animal and the theory states that it has very close similarities with REM sleep. One similarity is the total paralysis of the body. Other similarities include brainstem and changes in the thermoregulation of the body.
We Move our Eyes Because of our Dreams
Scanning hypothesis suggests that the movement of the eyes during REM sleep is triggered by the imagery of our dreams. The theory has a loophole since various experiments showed that people born blind and fetuses also experience rapid eye movement although they never had any imagery experience. Another fact that is against that theory is that the movement of the eyes during that phase. Finding that supports that theory is that if the dream is oriented towards a certain goal, the eyes are directed towards the action that occurs in the dream. This experiment was conducted in people who suffer from REM sleep behavior disorder and are able to reenact what they dreamt. They showed eye and body movements relating to the dream actions.
Discovery of REM sleep
The discovery of REM sleep phase came about in 1937. Richard Klaus, a German scientist, discovered that change in the brain activity while experimenting with cats. In 1944, the duration of the sleep cycle was founded. In 1952, a group of scientists discovered the peculiar movement of the eyes. It was immediately connected to dreaming.
In 1960, the work of William Dement was published. It explored the effects of REM deprivation.
Other experiments involving neurosurgery helped in the understanding of atonia. It also suggested the importance of dorsolateral pons regarding REM sleep. Other scientists found that the damage of reticular formation of the brainstem hindered the organism from going into REM sleep. In 1959, REM sleep became known as ‘paradoxical sleep’. It was named so by Jouvet who conducted experiments involving the removal of the entire forebrain of a cat and its consequences.