Introduction.

Contrary to popular belief, you are not unconscious while you sleep.  Sleep is simply a different state of consciousness!  All states of consciousness share some common features.  All arise from the electrical activity of the cerebral cortex.   All possess some degree of awareness of the surrounding world.  And finally, all can give rise to mental activity and behavior.  The mental activity and behavior of sleep are the subjects of this post.

One of the most important discoveries of sleep research is that, over the course of the night, we oscillate back and forth between 2  different types of sleep.  Since mammals share this type of sleep with birds and perhaps some reptiles, a version of this type of sleep likely appeared long ago in an ancestor to both mammals and birds.  A future post will address the evolution and adaptive functions of these 2 types of sleep.

So what are the two types of sleep?

The two types of sleep are Rapid Eye Movement (REM) Sleep and Non-Rapid Eye Movement (NREM) sleep.  Eye movements can occur in both, although less common in NREM sleep, and more rapid during REM Sleep.   The original sleep taxonomy developed by Rechtschaffen and Kales in 1968, identified a single stage of REM sleep and 4 different stages of NREM sleep.  However, the distinction between the two deepest stages of NREM sleep (NREM3 and NREM4) did not prove very useful.  So in 2007, the American Academy of Sleep Medicine combined the older NREM3 and NREM4 stages into a single NREM 3 stage.  The newer NREM3 stage, also referred to as Slow Wave Sleep, and is now considered the deepest and most important type of NREM sleep.  The use of EEG to define the different NREM stages is detailed in another post.

A hypnogram showing REM sleep and the different stages of NREM sleep over a typical night is seen in Figure 1. The lightest stages of sleep (NREM1 and NREM2) are thought to be the least important and serve mainly as transitions between  the more important stages.  The functions of the most important stages, NREM3 and REM, will be covered in another post.

Over the course of the night, the two different types of sleep alternate in approximately 90-minute cycles, with a single cycle containing a bout of NREM sleep (often containing multiple NREM stages) followed by a bout of REM sleep.  In a typical night, a person experiences 3-5 cycles. REM sleep accounts for around 20-25% of a night’s sleep, NREM3 (Slow Wave Sleep, Deep Sleep) for around 10-25% and light NREM sleep (NREM1 and NREM2) for around 50-60%.

It is interesting that the 2 most important sleep stages typically occur at different times of the night.  NREM3 sleep almost always occurs during the first third to half of the night’s sleep (see Figure 1) while REM bouts are the longest and most intense toward the end of the night’s sleep.

Despite the fact that almost everyone sleeps and dreams, there are a lot of misconceptions.  This post addresses a number of questions about dreaming and other sleeping phenomena.

What is the mental activity occurring during REM sleep?

Most folks know the answer to this, it’s dreaming.  A REM dream is a vivid, highly visual, hallucinatory, often emotional, and sometimes bizarre mental event that has a storyline in which the dreamer is an active participant.  Sometimes the dream’s storyline contains elements of the person’s recent experiences.  For many years it was thought that dreaming occurs only during REM sleep, however, we now know that dreaming, or something very much like it, occurs during NREM sleep as well.

How do we know when dreaming occurs?

That’s simple.  Have someone sleep in a sleep laboratory while measuring their brain waves.  Wake them during different sleep stages and ask “were you dreaming”?  If they were experiencing REM sleep, they will say “yes” around 80-90% of the time and will describe what we think of as a dream.  In fact, estimates are that 8o% of dreaming occurs during REM sleep.

If you awaken a sleeper during NREM sleep, and ask them if they were dreaming they will say yes around 10-20% of the time. However, if you probe deeper, you find that the likelihood depends upon the stage of NREM sleep.  Reporting a dream is most likely to occur in NREM1 and least likely in NREM3.  This suggests that perhaps some NREM dreams may originate in REM sleep and spill over into adjacent NREM1 sleep.  Perhaps NREM1 is best able to support dreaming because its cortical EEG brain waves are most similar to those of REM sleep.

How do REM dreams compare to NREM dreams?

There do appear to be some differences between between REM and NREM dreams.  REM dreams are typically rated as “more intense, bizarre, vivid, visual, and kinesthetically engaging” and the sleeper’s verbal descriptions tend to be longer with more continuous storylines.  On the other hand, the description of  NREM dreams tends to be shorter, less detailed, and more disconnected.  The differences have been described as analogous to seeing a collection of photographs versus watching a movie.  Yet other NREM “dreams” are even more different, more like the non-visual, conceptual thinking when awake.

There is not complete agreement as to whether the differences between REM and NREM dreams are qualitative or quantitative.  Consequently, we are not sure whether REM and NREM dreaming are caused by the same, or different, underlying brain processes.  Certainly more research is necessary here.

How long does a REM dream last?

For REM dreams, a dream typically lasts as long as a REM bout.  REM bouts can last anywhere from a few minutes up to 20 or 30 minutes.

However a single REM dream can have different “episodes” where one episode morphs into another. For example I remember a particularly vivid dream where in one episode I was laying on a lounge chair by a swimming pool and suddenly the lounge chair changed into a Harley Davidson motorcycle and I was speeding down the highway. Where that came from, I’ll never know since I have never owned a motorcycle, much less a Harley Davidson.  Probably my inner Walter Mitty breaking through!

Although the body is mostly immobile during REM sleep, occasional small body movements are thought to correlate with the switch from one episode to the next.

How fast does time pass in a REM dream?  

Some people feel that time moves faster during a dream, allowing them to experience more in a given amount of time than when awake.

The way this has been tested is to awaken sleepers at various times after entering a REM bout and asking them “how long have you been dreaming?”   Although individual estimates may not be exactly correct, if you take the average of a large number of estimates and compare that to the average of how long they had been in REM sleep, the averages are remarkably close. So the answer is that time appears to pass at about the same speed when dreaming as when awake.

How many times do we have a REM dream each night?

We normally have as many dreams each night as we have REM bouts, which is typically around 3-5.  All other things being equal, the longer you sleep, the more you dream.  The dreams (and REM bouts) usually become longer and more intense as the night progresses.

Do some people not dream?

Some people feel that they don’t dream.  However if such a person is put in a sleep laboratory, woken during a REM bout, and asked if they were dreaming, they almost always say, “yes.”  So, most people have multiple dreams every night, it’s just that some people are better at remembering their dreams.  At the same time, there are rare neuropathologies, and some drugs (e.g. stimulants and SSRI antidepressants), that can suppress the ability to experience REM sleep and dreaming.

Why are some people better at remembering their dreams?

There are likely multiple reasons. However one reason has to do with how you awaken in the morning.  If you wake up abruptly to a shrill alarm clock during your last REM bout of the night, which often contains the most intense, vivid dream of the night, you likely will be aware of that dream.  However, if you awaken gradually, you will likely not have a memory of your last dream.

Why are REM dreams so easy to forget?

Dreams are notoriously easy to forget.  If you wake a person one minute after a REM bout has completed, the person will often have no memory of the REM dream they were just experiencing.  Even when woken in the middle of a dream, if the dream is not quickly rehearsed, the details often seem to slip away.  One possible explanation is that dreams are encoded in transient short-term memory and not readily consolidated into the longer-term memory necessary for later recall.

However, my own experience, as well as that of others, would suggest this is not the case!  Some people can remember dreams from previous nights, often just before they fall asleep. In order for dream recall to occur days later, the dream must be encoded in long-term memory. An alternative explanation is that dreams are encoded as long-term memories, but for some reason, are difficult to retrieve into conscious awareness.

A possible explanation is that poor dream recall reflects state-dependent memory recall. That is, the ability to recall a memory can sometimes be enhanced if you can recreate the conditions under which the memory was originally acquired.  In so doing, you provide yourself with more cues for memory retrieval.  In this regard, the brain waves just before you fall asleep at night are very similar to those that occur during dreaming, perhaps enhancing dream recall.  State-dependent recall also accounts for why some of my college students prepared for exams by studying the night before in the classroom in which they will later take the exam.

In fact, some psychologists study sleep and dreaming, not because they are interested in sleep and dreaming per se, but because they are interested in the workings of memory.

What are Lucid Dreams?

A lucid dream is one in which the dreamer is consciously aware they are dreaming as the dream is occurring.  Some lucid dreamers are even able to influence the content of their dream, although this is not necessary for a lucid dream to occur.  In a large survey, about half the participants reported having a lucid dream at least once in their life and about a quarter did so on a regular basis.  People who practice meditation are more likely to have lucid dreams.  While lucid dreams are rare for most individuals, there are techniques that can be employed to increase their likelihood.

There is disagreement as to whether lucid dreams are good or bad for you.  On the positive side, lucid dreamers are usually better able to remember their dreams.  To the extent that dreams involve working through the problems of the day, perhaps this is good for mental health.  And for those lucid dreamers that can control the content of their dreams, lucid dreaming presumably should result in fewer negative dreams or “nightmares.”

On the other hand, lucid dreaming is an abnormal state, mixing elements of both being asleep and awake.  Most of the awakenings over the course of the night occur during REM sleep (see figure 1).   Since the cortex is neurologically more active during lucid dreaming, this type of dreaming may result an increased probability of waking up and of less sleep over the course of the night.  Since certain types of memory consolidation are also thought to occur during REM sleep, increased episodes of waking up might also impair memory consolidation and emotional regulation.  And finally lucid dreaming is common during a sleep pathology called cataplexy.  More about that later.

What are the Rapid Eye Movements doing?

The function of rapid eye movements during dreaming is controversial.  One school of thought is that the eye movements relate to the visual experiences of the dreamer.  We often interact with others in our dreams and the eye movements of REM sleep bear some similarity to the saccadic eye movements during awake social interactions.

However, other findings are not supportive.  For example, the eye movements while dreaming are less rapid that those of awake individuals.  In addition, there is some evidence that the electrical activity giving rise to rapid eye movements (PGO waves) originates in the brainstem before spreading to the cortex.  PGO stands for “Pons-Geniculate-Occipital” reflecting that these waves can be detected in a variety of brain areas.  While PGO waves can be detected in cats and rats in the part of the motor cortex that controls eye movements, they can also be measured in other brain areas. (Measuring PGO waves deep in the brain is too invasive to perform experimentally in humans.)  Consequently it is  possible that rapid eye movements are caused by brain areas outside the cortex and only indirectly related to the dream content generated by the cortex.

So whether the eye movements are related to the experiences in our dreams or are caused by unrelated, but correlated, neural activity in the brainstem is unclear.

What does the average person dream about?

Several sleep researchers have compiled books of dreams by waking up sleepers and asking them to describe their dream. The average dream content tends to be different from what most people would guess.  Most dreams are rather mundane, occurring in environments with which the person is familiar, populated largely by people that the dreamer knows.  Family, friends, money, work, school and health issues are often involved.  Dreams often reflect the person’s concerns, with the content more likely to be negative than positive.  Being chased, falling, arguing, being unable to prevent something from happening, and forgetting to attend important life events, are common themes.  There are also sex differences, with male dreams more likely to contain aggression and negative emotions.

In contrast, if you ask a person to describe one of their past dreams, they may tell you about their wonderful experiences being marooned on a desert island with Robert Redford or Jennifer Anniston (I’m dating myself here). Although there are many more mundane than exotic dreams, people are more likely to remember the interesting ones.

What is the function of dreaming?

That is the $64,000 dollar question!  While there is much speculation, the answer is, “we don’t really know.”  A somewhat easier question, addressed in a future post, concerns the function of REM sleep.  However, even that does not have a definitive answer.

What happens when the neural mechanisms controlling sleep malfunction?

For most of us, sleep functions as it should most of the time.  However, sometimes the underlying neural mechanisms “misbehave” giving rise to parasomnias.  Parasomnias are sleep disturbances that take the form of abnormal behaviors, movements, emotions, and perceptions  during sleep. Neural malfunctions during both REM sleep and NREM sleep can give rise to parasomnias.

What happens when REM sleep malfunctions?

While experiencing a dream, primary motor cortex is attempting to send messages to your muscles that would cause you to act out the behaviors you’re performing in your dreams.  Fortunately, there is a brainstem mechanism that prevents that from happening by deactivating the 𝛼-motoneurons that would carry this message from the central nervous system out to the muscles.  With the exception of eye movements and breathing, you are almost completely paralyzed during REM sleep.  If this paralysis didn’t occur you would get up and act out your dreams.  This outcome was first confirmed a number of years ago by experimentally disabling the paralysis mechanism of cats.  By watching cats act out their dreams we know that cats dream about about “cat-like” things: eating, meowing, scratching, hissing at another cat, etc. 😀

In humans, occasionally some motor information  “sneak throughs” this protective mechanism, causing the twitching seen during REM sleep.  In fact, once or twice a year, even normal individuals may wake themselves during a bad dream by jerking an arm or leg.

REM Sleep Behavior Disorder (RBD).  However, a rare but more serious parasomnia is called REM Sleep Behavior Disorder (RBD) in which the paralysis mechanism fails, several times per month is not uncommon, resulting in the person attempting to act out their dream.   Since negative dream content is common, a sleeping person experiencing a RBD episode could start screaming, using foul language,  flailing an arm in anger, or even getting up and smashing into something.  Since they are asleep with their eyes closed, they usually don’t get very far.  However, the person can potentially hurt their sleeping partner or themselves.  On the other hand when experiencing a positive dream, the person might sit up smiling or laughing.  Most RBD episodes occur in the latter part of the night when REM dreams are typically the longest and most intense.  RBD is most common in middle-aged men although  it can occur in both sexes and at other ages.

A sleep test in a sleep laboratory is often necessary to diagnose RBD since there are other parasomnias, such as sleep walking (a NREM disorder), with overlapping symptoms.  There are steps that can be taken to minimize the likelihood of the RBD individual hurting themselves or others.  While some medications can help, there is no cure for this parasomnia.  Good sleep hygiene can help to control RBD, and is helpful for  all the other parasomnias as well.

Narcolepsy and Cataplexy.  Narcolepsy is another REM-sleep related parasomnia characterized by excessive daytime sleepiness.  There are two types: Type 1 and Type 2.  Both are relatively rare, with Type 1 being around 3 times more common than Type 2.  Neither is “curable” although their symptoms can be treated.

Type 1 narcolepsy, in addition to causing sleepiness, causes cataplexy.  During a cataplexy episode, an awake person, going about their daily activities, suddenly enters REM sleep.  When this happens, the person is instantly paralyzed and collapses like a sack of potatoes.  The fall potentially can cause injury or even death.  The episodes typically last from several seconds up to several minutes and are usually triggered by emotional arousal.  Anger, excitement, laughter, and even sexual arousal can precipitate episodes.  A cataplectic attack involves elements of both being asleep and being awake.  During the attack, the person often experiences a lucid REM dream while also retaining some awareness of their situation.  The person usually knows their “triggers” and is sometimes able to prevent, or minimize the consequences of, an attack.  A young woman filmed her own cataplexy attack on YouTube  to educate others about the disorder.

In some ways cataplexy is the opposite of RBD.  In RBD, REM-sleep paralysis doesn’t occur when it should,  while in cataplexy, it occurs when it shouldn’t.  There is considerable variability in age of onset, with the mean being around 24.  The disorder sometimes exhibits progressive onset with the initial symptoms being just excessive sleepiness before the cataplexy develops.

Type 1 Narcolepsy is most often caused by a severe deficiency in the hypothalamic neurotransmitter orexin (also called hypocretin).  The orexin-releasing neurons selectively die off although what causes them to die isn’t known.  There is an association of this disorder with a genetic allele influencing immune functioning suggesting that Type 1 Narcolepsy might have an autoimmune cause.  Type 1 Narcolepsy does run in families supporting a genetic predisposition, although environmental factors also play a role.  For example, the symptoms often first appear after an illness, such as a cold or flu.  The occurrence of Type 1 Narcolepsy in other animals, including mice and dogs,  has been useful to scientists in better understanding the disorder.

Type 2 Narcolepsy manifests as simply a low threshold for falling asleep.  The individual does not suffer from cataplexy or have a loss of orexin-releasing neurons.  It is less common than Type 1, and the symptoms generally less serious.  However, as with Type 1 narcolepsy, the excessive sleepiness can negatively affect school and job performance and can sometimes be dangerous.  For example, individuals with either type of narcolepsy are around 3 to 4 times more likely to be involved in car accidents.

For both Type 1 and Type 2 narcolepsy, the symptoms are often exacerbated by sleep deprivation.  Engaging in good sleep hygiene and taking naps during the day can be helpful.  Treatment can also involve drugs that promote wakefulness, and for Type 2 narcolepsy, drugs that inhibit REM sleep.

Sleep paralysis.  A less serious parasomnia of REM sleep escaping into wakefulness, is sleep paralysis.  This disorder happens either just before falling asleep or just after waking up, and finding that you are “paralyzed” for a period of time. Like cataplexy, sleep paralysis represents a mixed state of consciousness with elements of being both awake and asleep.  During a paralytic attack, the person often experiences a REM dream, often with negative content, superimposed on their waking consciousness.  The episodes can last from a few seconds up to as long as 20 minutes, with the average length being around 6 or 7 minutes.  Although this disorder can also occur in narcoleptic individuals, most cases are not connected.

Around 8% of the population will experience sleep paralysis at least once in their lives, but for some, it occurs on a regular basis.  However, even when occurring regularly, sleep paralysis can be only an annoyance that doesn’t adversely affect the quality of the person’s life.  I had a very good student in one of my classes who also won an NCAA national championship in wrestling who said he had this issue several times a week.  While he would rather not have the issue, he said he was not particularly affected by it.   He told me others in his family also had sleep paralysis.

However, for around 10% of sufferers, sleep paralysis is troubling enough to seek treatment.  The treatment involves following good sleep hygiene, while behavioral therapies and certain drugs can also help.

Are there any problems during NREM sleep?

There are also parasomnias that occur during NREM sleep including 1) sleep walking 2) sleep terror, and 3) confusional arousal.  These problems are most common in childhood and usually resolve by puberty, although they can occur in adulthood as well.  Often these problems are associated with issues that disrupt sleep such as sleep apnea, night-time leg cramps, and poor sleep hygiene.  Treating these other problems and promoting good sleep habits can be beneficial in controlling all parasomnias.

Sleep walking.  Sleep walking, sometimes referred to a somnambulism, occurs in around 30% of children between the ages of 2 and 13 and around 4% of adults.    Sleep walking occurs primarily early in the night’s sleep during NREM3 sleep,  The individual will get out of bed and become behaviorally active.  The behaviors could include such things as walking around, getting dressed, moving furniture, urinating in inappropriate places, and sometimes even more complex behaviors such as driving a car.  In rare cases, sleep walkers can engage in violent behaviors overlapping those of REM sleep disorder.  In this case, testing in a sleep laboratory may be necessary to distinguish these disorders.

The episodes can last from a few seconds up to 30 minutes with an average of around 10 minutes.  Often the episode ends with the person returning to bed without waking up with no memory of the episode.  If the person wakes up before returning to bed, they are very disoriented and confused, with no knowledge of how they got where they are. Because these episodes often go undetected by others, it is difficult to estimate their frequency.  There are potential dangers.  A person can hurt themselves by falling, colliding with objects in the environment, or by using dangerous objects such as knives or guns.  They can also potentially harm a sleeping partner.

For the most children, the episodes are rare, resolve on their own, and typically don’t require treatment.  However for individuals that sleep walk regularly, there are steps that can be taken to minimize harm. Locking windows and doors of the bedroom, installing motion sensors and bed alarms that are triggered when the person gets out of bed and removing potentially dangerous objects.  Sleep deprivation and stress are often associated with sleep walking, and good sleep hygiene helps in preventing episodes. Cognitive behavioral therapy can also be helpful.  Medications that promote NREM3 sleep can make the problems worse and should be discontinued.

If you encounter a sleep walker, the best strategy is to try to guide them back to their beds.  However, if you need to awaken a sleepwalker you should do so carefully as the person will be disoriented, confused, and perhaps frightened, and may have trouble getting back to sleep.

Night Terror.   Another NREM-related parasomnia is called a night terror (sometimes called a sleep terror).  As with other NREM-related disorders, night terrors are most common in children and typically resolve by puberty.  When a night terror happens, the child will often sit up in bed screaming, exhibiting signs of an intense “fight or flight” reaction, including increased heart rate, fast breathing, and covered in sweat.   However, the child does not acknowledge attempts at consolation, which can be very distressing to the parent.   After a few minutes, the child typically lays back down and sleeps normally for the rest of the night.  The next morning there will be no memory of their night terror.

A night terror is not the same thing as a “nightmare.”  A nightmare is an unpleasant REM dream typically occurring in the second half of the night’s sleep.  Night terrors, on the other hand,  occur during  the first third to half of the night in the deepest stage of NREM sleep (NREM3).  While screaming and yelling are common during night terrors, there are no vocalizations during a nightmare. Furthermore, nightmares are often remembered the next day, while night terrors are almost never remembered.  And finally while virtually all children have nightmares, night terrors are less common, occurring in less than 7% of children.

Although the cause is unknown, night terrors (as well as sleep walking and confusional arousal) are more common in close relatives, indicating a genetic predisposition. Many children may have only a single night terror in their childhood before outgrowing them. However, for some children they can occur regularly.  Parents should not wake the child during a night terror.  The child will be disoriented and confused and will most likely take longer to get back to sleep.

Like sleepwalking, the occurrence of night terrors is often associated with stress and a lack of sleep.  For example, a little over half of the children seeking treatment for night terrors also suffer from obstructive sleep apnea.  Other disorders that disrupt sleep, such as asthma, restless leg syndrome, or gastrointestinal reflux disorder, are also associated with night terrors.  Consequently, treating the associated disorders, as well as promoting good sleep hygiene and minimizing stress is often therapeutic.

Confusional Arousal.  Another disorder of NREM3 is confusional arousal.  Here the person sits up in bed and begins talking with a vacant stare, although unlike sleep walkers, they typically do not get out of bed and walk around.  What distinguishes this disorder from night terrors is the person doesn’t appear terrified and does not show sympathetic arousal.  However, their speech patterns are similar to those of an intoxicated person, being slow and halting and characterized by confusion and disorientation.  Consequently this disorder is sometimes referred to as “sleep drunkiness.”

If you try to interact, the person most often doesn’t engage.  If the person does respond, the response can sometimes be aggressive or hostile.   If not awakened, the person typically goes back to sleep and has no memory of the episode the next day.  As with other NREM3 sleep disorders, confusional arousal is most common in children.

Although considered different from sleep walking or night terrors, confusional arousal can evolve into those disorders over time suggesting that all 3 share underlying causes.  The treatments are also similar, including promoting good sleep hygiene and reducing stress.

Concluding Remarks.

Hopefully I’ve convinced you that your sleeping brain is not quiescent and that there’s a lot of stuff going on.  And sometimes these complex neural processes malfunction.  The next post will examine the neuroanatomy and neurophysiology that causes us to fall asleep each night.