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Sleep is sometimes called the forgotten third of existence. It is an altered state of consciousness we experience–and mostly forget–nearly every night.

For the first half of the 20th Century, sleep seemed to be forgotten by psychological researchers as well. A typical psychology textbook of the 1920s through 1940s devoted only a few paragraphs to the subject of sleep.

This changed when REM sleep was discovered. Information about sleep and dreaming accumulated at a rapid pace during the 1950s and 1960s. That era became known as the Golden Age of Sleep Research.

The Discovery of REM sleep

The Golden Age of Sleep Research began in 1952 with observations of body and eye movements during sleep. The research was carried out by a graduate student, Eugene Aserinsky, in a pio­neering sleep laboratory at the University of Chicago.

Aserinsky recorded movements of sleepers on film. As he wrote in his 1953 dissertation:

A new type of eye movement was discovered to occur in the sleep of adults and a child but not in infants. Motion pictures confirmed the presence of these eye movements, which were binocularly synchronous, rapid and jerky. It was suggested that they be termed "rapid" eye movements in contrast to the slow eye movements previously reported. (Aserinsky, 1982, p.1274)

Rapid eye movements (REMs) are very rapid: up to 8 movements back and forth per second. They look like tremors or vibrations of the eyes, not normal eye movements. They occur in bursts sometimes called REM storms.

Aserinsky soon realized REMs occurred during a distinctive phase of sleep known since the 1930s. During this phase of sleep, a person's brain waves (electro­encephalograph or EEG recordings) show an active alert pattern.

This EEG pattern is identical to the EEG of a person who is awake and thinking. Aserinsky wondered if that indicated the sleeper was dreaming. Sure enough: people awakened during the REM stage reported dreams 70 to 80 per cent of the time.

How does the EEG look during REM sleep? What happened when people were awakened during REM sleep?

The phrase REM sleep refers to more than just eye movements. REM sleep is a distinctive stage of sleep identified by many criteria.

The eyes do not move constantly during REM sleep. Sometimes there are 5 minutes between bursts of REMs while a person remains in REM sleep by other criteria. During REM sleep there is an aroused EEG pattern, muscle relaxation below the neck, fluctuating heart rate and rapid changes in breathing rate.

Human males have an erection during REM sleep. Women have increased vaginal blood flow during REM sleep. This generally has nothing to do with sexual content of dreams; it is part of the overall REM sleep pattern.

How does non-REM sleep contrast with REM sleep?

During the other type of sleep, called non-REM sleep, the EEG shows large, regular waves. Breathing and heart rate are slower and more regular than during REM sleep. During non-REM sleep large, slow eye movements occur.

REM Sleep in Cats

The discovery of REM sleep ultimately had a huge impact on sleep research. However, the initial report of a connection between REM sleep and dreaming was met with "an outburst of apathy" in the scientific community (Lubin, 1974).

Many psychologists were skeptical about any report involving a pop psychology topic like dreaming. (Pop psychology is a negative label for topics discussed in unscientific ways in popular magazines).

Only after REM sleep was demon­strated in animals, years later, did psychologists awaken to its signif­icance. If REM sleep occurred in cats as well as humans, it must be real!

William Dement's 1958 discovery of REM sleep in cats, plus ground­breaking work by French physiologist Michael Jouvet (zhu-VAY), really started the Golden Age of sleep research.

Why did the discovery of REM sleep in cats have a big impact?

Jouvet did research on sleep mech­anisms in cats. When Dement published a report of REM sleep in cats, Jouvet realized he had seen the same thing. Jouvet called it paradoxical sleep.

Jouvet chose the term "paradoxical" (which means strange or contra­dictory) because during this phase of sleep animals showed biological signs similar to an awake animal. Breathing rates fluctuated, heart rates went up and down, and their eyes made quick, jerky movements beneath closed eyelids.

The body of a cat (or a dog) in para­doxical sleep appears deeply relaxed. However, traces of activity appear all over their bodies. There are tremors in the paws, quick scratching movements and rapid twitching of the whiskers. Their EEGs show a noisy "alert" pattern.

Why did Jouvet call REM sleep "paradoxical sleep"?

Animals were sometimes difficult to awaken from REM sleep. Despite signs of activation such as twitching paws, the animals were never in an upright posture. When they went into this phase of sleep; they went completely limp, as if major muscles of the body were paralyzed.

Jouvet had the same insight Dement and Aserinsky did. The brain and muscle activity might indicate dreaming. If so, deep muscle relaxation might be necessary to prevent the animals from acting out their dreams.

Jouvet found that a brain area called the locus coeruleus, near the pons in the midbrain, was necessary for muscle relaxation during REM sleep. When this area was surgically destroyed, the animals went to sleep normally.

However, during paradoxical or REM sleep, the cats hissed and scratched violently. Sometimes they ran around the cage with their eyes closed, as if attacking another animal.

Jouvet (1967) reported, "The sleeping animal's behavior may even be so fierce as to make the experimenter recoil." Evidently the cats were acting out dreams about hunting, or perhaps fighting off rivals.

What happened when Jouvet destroyed the locus coeruleus?

Jouvet's research implied that animals dream, just like humans. It also suggested that muscle relaxation below the neck during REM sleep prevented animals from acting out their dreams.

The transition from non-REM to REM sleep can be observed in cats. In non-REM sleep, a cat's muscles are active, so it can sleep upright or in a sitting position as in the sketch below.

can sleeping upright in the meatloaf positionA typical slow wave sleep posture

An EEG shows large, slow waves while a cat sleeps in this position. By contrast, when a cat enters REM sleep, it loses all muscle tension below the neck and relaxes completely, as shown below.

When a cat or other animal is limp like this, but its paws are twitching, the EEG shows rapid, erratic activity characteristic of REM sleep. We can assume the cat is dreaming at that time.

a cat asleep on its sideMandy during REM sleep

In dogs, cats, and rabbits (among other animals) one can observe external signs of dream activity such as paws and whiskers twitching during REM sleep. Mandy's paws were twitching when the above picture was taken.

How can you tell from a cat's posture whether it is in REM sleep?

Relaxation of posture muscles can sometimes be observed in students. If a student goes to sleep during a lecture, the posture muscles retain their tension at first, so the student remains upright.

In a few minutes the student may move into something like REM sleep. The muscles relax, and the student gradually keels over, striking the desk or a nearby classmate. The student usually wakes up confused, having been awakened in the middle of a dream.

One student reported that he avoided falling asleep in lecture by grasping a pencil between two fingers when he felt drowsy. If he started dropping off to sleep, the pencil slipped from between his fingers, which woke him up. Perhaps if a lecturer hears a clattering of pencils dropping to the floor, it is time to move on to the next topic.

How did a student keep himself from falling asleep in class (and why would this work?)

After Jouvet's research, other scientists looked for evidence of paradoxical or REM sleep in animals of other species. It turned out to be very common, especially among mammals.

In fact, periods of REM or paradoxical sleep occur in all mammals except the spiny ant-eater (echidna), which is not a typical mammal because it lays eggs. Among mammals, predators spend the most time in REM sleep.

What mammals have the most and least REM sleep?

Allison and Cicchetti (1976) suggested that prey animals have less REM sleep because they need to wake up quickly if a predator finds them. Therefore they spend more time in lighter stages of sleep.

Stages of Sleep

Sleep researchers distinguish between several different phases of sleep. They follow each other in a cycle repeated three or four times per night.

Each complete cycle takes about an hour and a half. A group of researchers proposed in 1937 that the cycle be divided into 5 stages identifiable in the EEG record, labeled A through E.

Now these stages are labeled 1 to 4, with the fifth being REM sleep. Some researchers call wakefulness before sleep stage 0.

During Stage 0 a person is still awake but sleepy. The EEG shows mostly alpha waves during this time: waves in the 8-12 per second frequency range.

Alpha waves are normally blocked by eye movements. If alpha waves are present when a person is lying down, it indicates the eyes are being rested, but the individual is still awake.

What is stage zero?

Stage 1 sleep (the earliest stage) marks the onset of sleep. It is characterized by the disappearance of alpha waves and a cut-off of attention to the environment.

The cessation of attention to the environment is sudden and complete. This can be studied by taping subjects eyes open when they are very sleepy. They are instructed to press a button each time a light flashes.

Subjects will suddenly stop pressing the button when they enter Stage 1. The light continues to flash into their still-open eyes, but as soon as they enter Stage 1, they cease to respond to it.

What are distinctive characteristics of sleep onset?

The switch of attention from outside to inside is often accompanied by odd thoughts or imagery that are distinctly dream-like. That is called the hypnagogic state.

At the same time, the alpha waves in the EEG disappear. Independent observers can agree when sleep onset occurs, 80% of the time, using only EEG records.

Soon after a person falls asleep, stage 2 of non-REM sleep occurs. It is identified by two distinctive EEG patterns called sleep spindles and K-complexes.

During the first few sleep cycles of the night, stage 2 is followed by stages 3 and 4, before a REM period. Later in the night, people tend to go directly from stage 2 into REM sleep.

Stages 3 and 4 of non-REM sleep are identified by increasingly prominent brain waves on the EEG called delta waves. Delta waves are large, slow waves caused by the synchronized firing of many neurons.

Slow waves are presumably caused by neurons in a resting pattern, pulsing together. This phase of non-REM sleep is called slow wave sleep or delta sleep.

If deepness of sleep is defined by how hard it is to awaken a creature, stage 4 is the deepest stage of sleep for humans. In other animals, the REM state is the deepest state (producing the slowest awakening).

What is "slow-wave" or "delta" sleep? Which stage of sleep is the "deepest" for humans?

People describe a good sleep as sound, which means "solid and unmoving." As the expression suggests, the type of sleep that feels deepest to humans is a sleep with little body movement. Usually this indicates lots of stage 3 or stage 4 sleep.

Tossing and turning accompanies lighter sleeps with more time in stages 1 and 2. People often get all the delta sleep they need during the first two sleep cycles.

Sleepers tend to alternate between stage 2 and REM sleep after that. Often dreams become deeper and more involving late in the sleep process, as more time is spent in the REM phase.

sleep stages

Above is a classic diagram of sleep stages during one night (after Dement & Kleitman, 1957). The first REM period occurs after about an hour.

Note that REM periods are usually entered from stage 2, but the first are more likely to be entered from stage 4. The REM periods last from 2 to 30 or more minutes.

When does the first true REM sleep period usually occur? How long does it last?

During REM sleep, an EEG shows an arousal pattern similar to the pattern shown by an awake brain. At the same time, muscle tension below the neck almost disappears. Breathing and heart rate fluctuate.

After the REM sleep period, the sleeper typically returns to stage 2 sleep, and the cycle of non-REM to REM sleep repeats again. A single sleep cycle consisting of non-REM stages followed by an REM period averages 90-100 minutes in length. Typically, three or four REM periods occur each night.

Attempts to Explain Sleep

Scientists are unsure why we sleep. There may be many reasons. One is the advantage of staying hidden during vulnerable times. For primitive humans, nighttime was more dangerous than daytime. For nocturnal animals, the day is the time to hide.

Perhaps the underlying reason for sleep is the radical difference between day and night. Most animals are specialized for darkness or light but not both.

What are some theories about why animals sleep?

Sleep may help the body recover from extremes of activity. Marathon runners double their slow-wave (non-REM) sleep after running a marathon.

However, sleep is not a rest for the brain as a whole. The brain consumes as much energy during sleep as it does when we are awake. It consumes slightly more than usual during REM sleep.

Is there a chemical that accumulates in the bloodstream while we are awake, causing us to become sleepy? Research­ers have looked one since Henri Pieron showed in 1913 that blood transfused from a sleeping animal made a second animal sleepy.

Pieron theorized that the blood of a sleepy animal carried a chemical which he called a hypnotoxin. It would be cleared out of our systems as we rest. That would explain why we feel a need for sleep, and why we feel better afterward.

A chemical that seems to act like Pieron's hypnotoxin is the transmitter adenosine. Adenosine accumulates when animals are awake. Adenosine levels fall during sleep until the animal wakes up again (Rainnie, Grunze, McCarley, & Greene 1994).

In Chapter Two (Human Nervous System) we saw another clue implicating adenosine. Caffeine increases alertness, and caffeine blocks adenosine. Caffeine occupies the receptor sites where adenosine would normally go, preventing adenosine from exerting its usual soporific effect.

Experimental studies from 2008 to 2017 suggested that sleep might aid learning and memory by deleting unused con­nections. Synapses that are unneeded can be thinned out, while others are strengthened.

Vivo et al. (2017) studied 6920 synapses in the mouse motor and sensory cortices using three-dimensional electron microscopy. The interface between axons and dendritic spines, forming synapses, was reduced by an average of 18% after sleep. This shrinkage was selective, sparing synapses that were larger and more stable.

The researchers concluded that a core function of sleep was to "renormalize" overall synaptic strength, preserving stronger synapses that were truly useful, while reducing the volume of others. This process allows overall volume of the brain tissue to be preserved.

Normal Sleep

Many people have heard that 8 hours of sleep is normal. That is an average, but there are wide variations.

In one study, 4000 entering freshmen at the University of Florida filled out ques­tionnaires asking about sleep habits. Fewer than 50% averaged between 7.5 and 8.5 hours of sleep per night.

20% slept less than 7 hours, 5% slept more than 9 hours on the average. The overall average was 7.2 hours per night (Webb, 1981).

One way to compensate for loss of sleep is to take short naps. Leonardo da Vinci used this technique when staying up for long periods of time: he would catch a 15-minute nap every four hours.

College students also nap frequently, according to a University of Florida study described by Webb (1981), "The amount of napping was far greater than expected."

Only 16% did not nap in a two-week period. The average bedtime was 12:45 a.m., and the average wakeup time was 8:25. There were no significant male/female differences.

What unexpected finding turned up in research on U of Florida freshmen?

For optimal napping, one approach is to exploit natural rhythms of the 90 minute sleep cycle. For the first 15 minutes of the cycle, typically, sleep is light, yet one can dream.

Dreaming, even briefly, seems to result in feeling refreshed. The minutes immed­iately after going to sleep are one of the few times people report REM-type dream content, outside of REM sleep, so even a brief nap can result in dreaming.

Many students assert that a longer nap can be counterproductive. After half an hour, or 45 minutes, a napper may enter the deeper stages of sleep. If forced to awaken, he or she may feel more tired than before.

After a full 90 minute cycle, a sleeper is typically close to the surface again. Body temperature is higher and movements are more likely to occur. In a person with regular sleep cycles, getting up after any multiple of 90 minutes is relatively easy, compared to other interruptions of sleep.

So–bottom line–plan for a 15 minute nap, or plan for some multiple of 90 minutes. If your sleep rhythms are typical, this maxi­mizes the likely benefits of napping.

Age-Related Changes in Sleep

A persistent myth about sleep duration is that people need less and less sleep as they age. Observational studies show no drop-off in sleep required, as people age. However, the number of reported nighttime awakenings goes up drama­tically with age.

That is the only consistent difference in sleep patterns of old and young people. At all ages there are wide variations between individuals. For example, in a sample of 70 year olds, the average amount of sleep per night ranged from 5 hours to 12 hours.

How do sleep patterns of young and old differ?

Research indicates that teenagers take an average of 15 minutes to fall asleep. Very rarely does a person take 30 min­utes to fall asleep, even in the unfamiliar environment of a sleep laboratory.

However, over a quarter of 11-16 year olds think they take more than half an hour to get to sleep, four or more nights a week (Webb, 1981). Evidently they do not notice when sleep actually starts.

Thomas Roth, a psychologist at Henry Ford Hospital in Detroit, found that about 25% of healthy 18-to-35 year olds fell asleep in less than six minutes. The other 75% normally took longer.

However, "if subjects who normally took longer than six minutes to fall asleep were deprived of one hour of sleep a night for five nights, they fell asleep in less than six minutes as well" (Adler, 1993). In other words, one week of moderate sleep deprivation–an hour a night, for a week–made everybody fall asleep very quickly.

How long does it take the average teenager to fall asleep? How can young adults fall asleep in less than six minutes?

William Dement was the Stanford University sleep researcher who helped start the modern era of sleep research. He believed that teens needed up to 10 hours of sleep per night. Dement argued that early schedules required by high schools in America, many starting shortly after 7 a.m., created major problems.

"It is not uncommon to look at a high school classroom in the morning and see one-third of the students with their heads on their desks," he pointed out. He suggested that school districts move starting times to later in the morning.

In 1998, Representative Zoe Lofgren (D-CA) introduced a bill that would grant any school district in the United States $25,000 to rearrange high school schedules so kids could sleep later. However, in the few school districts where this was proposed, the idea was rejected as too inconvenient or expensive.

A few tried it but found problems moving the whole school day forward. That cut into afternoon time normally set aside for activities like sports, valued by students and parents ("Combating student torpor," 1998).

Several school districts did try later schedules. Morgenthaler et al. (2016) attempted a meta-analysis of studies showing the effect of moving high school to one hour later. They found that "often the evidence is indirect, imprecise, or...very weak."

Out of 287 candidate publications, 8 studies were found to be suitable for detailed comparison. In those studies, "We found that later school start times, particularly when compared with start times more than 60 min earlier, are associated with longer weekday sleep durations, lower weekday-weekend sleep duration differences, reduced vehicular accident rates, and reduced subjective daytime sleepiness." (Morgenthaler et al., 2016)


Allison, T. & Cicchetti, D. (1976). Sleep in mammals: Ecological and constitutional correlates. Science, 194, 732-734.

Adler, T. (1993, September). Speed of sleep's arrival signals sleep deprivation. APA Monitor, 24, p.20.

Adler, T. (1993, September). Sleep loss impairs attention - and more. APA Monitor, 24, pp.22-23.

Combating Student Torpor. (1998) Science, 281, p.39. doi:10.1126/science.281.5373.39c

Aserinsky, E. (1982). Eye Movement Patterns in Infants. Science, 215, 1274.

Dement, W. & Kleitman, N. (1957) The relation of eye movements during sleep to dream activity. Journal of Experimental Psychology, 53, 339-346.

Haimov, I. & Lavie, P. (1996) Melatonin--A soporific hormone. Current Directions, 5, 106-111.

Jouvet, M. (1967, February). The states of sleep. Scientific American, pp.62-72.

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Morgenthaler, T. I., Hashmi, S., Croft, J. B., Dort, L., Heald, J. L, & Mullington J. (2016) High school start times and the impact on high school students: what we know, and what we hope to learn. Journal of Clinical Sleep Medicine, 12, 1681-1689.

Rainnie, D. G., Grunze, H. C. R., McCarley, R. W., & Greene, R. W. (1994) Adenosine inhibition of mesopontine cholinergic neurons: Implications for EEG arousal. Science, 263, 689-692.

Vivo, L. d., Bellesi, M., Marshall, W., Bushong, E.A., Ellisman, M.H., Tononi, G., & Cirelli, C. (2017) Ultrastructural evidence for synaptic scaling across the wake/sleep cycle. Science, 355, 507-510. doi:10.1126/science.aah5982.

Webb, W. (1981). Sleep Disorders and Modes of Treatment. Riverside, CA: Psychological Seminars, Inc.

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