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The Enchanted World of Sleep

by Peretz Lavie

Yale University Press, 1996. First published in Hebrew in 1993 by Yedioth Ahronoth Books, Tel Aviv, Israel. Available in paperback from amazon.com.

Why do we sleep?

Aristotle thought that sleep and wakefulness are results of our ability to feel and comprehend the stimuli of our environment. You can't sleep (or be awake) if you don't have sensory organs.

"Voltaire placed sleep in the same level as hope." (Axel Munthe, The Story of San Michele)

Sleep is linked to growth and to hormone secretion.

Prolonged sleep deprivation can cause psychosis. Rats will die after 7 to 10 days of sleep deprivation, 40 to 60 days after deprivation of the REM stage only.

Sleep is essential for fundamental physiological mechanisms, such as temperature regulation.

Daily hormone secretion peaks in adults and children in sleep stages 3 and 4. Further research has indicated that sleep and secretion are two separate mechanisms that are closely linked.

Cortisol is secreted under stress; it mobilizes the metabolism. It reaches its daily peak at awakening. The secretions are not gradual, but occur in spurts.

Cortisol appears to be controlled by an independent biological clock which is not always coordinated with the sleep clock.

Other hormones are tied to the sleep clock: gonadotropic hormones are released during sleep in spurts from the pituitary gland every 90 minutes.

The pulse delivery is crucial; if released continuously, it will inhibit growth of gonads.


Sleep trains the neural networks which are related to instinctive behavior. (Dr. Michel Jouvet)

During REM neural networks are activated independently of the muscles, .."similar to a trial run of a giant computer program conducted to ascertain that the computer is in working order...the neural networks linked to instincts are checked every night."

REM sleep may play a vital role in the maturing of the nervous system.

It also appears to be the solution to the problem of uniting brief periods of sleep into a single continuous one.

Are dreams simply a wonderful, but unessential byproduct of the aroused of brain during REM? (Lavie)

Lavie's patient, Mr. Y. has a splinter of shrapnel right in the REM center (the pons) of his brain stem. Exhibits no traces of REM sleep, yet he learns and functions normally. Is REM only essential for infants? Maybe, but intact adult rats die from selective REM sleep deprivation. And sleep pattern of intact mammals after short-term REM deprivation show increasing REM sleep as % of total sleep

REM phases are "gate to wakefulness" during sleep, allowing animal to scan its environment.

"Gates to sleep" occur during early morning hours and between 4 pm and 6 pm in the evening.

The same year that Lavie examined Mr. Y., Francis Crick and Graeme Mitchinson hypothesized that REM clears out our memory banks. (Nature 1983)

They proposed that REM sleep organizes our neural networks, and that dreams are composed of items destined for removal from the memory banks. Without this nocturnal erasure the vast amounts of information that reach the brain every day would jam it.

[Equally plausible is the notion that most of what our senses report is never stored in the first place.]

REM helps us acquire motor and perceptual skills. (Lavie)

REM sleep helps in learning certain "higher order" knowledge, such as new languages.

REM of endogenously depressed patients is unusually prolonged and turbulent. Prevention of REM can relieve symptoms, as can electric shock.

What Signals Sleep?

The brain stem.

REM sleep and deep sleep are controlled by different areas in the brain stem, each of which are rich in different neurotransmitters.

The Environment

Light, as well as learned environmental stimuli, can cue these centers.

Selected deprivation of REM sleep in people [with REM center intact] is difficult to accomplish. After a while, if you try to prevent REM sleep, you have to prevent all sleep.

Distinctive brain waves, muscle tonus, and eye movements signal the various stages of sleep.


Gross body movements appear to be the "punctuation" between some sleep stages.

The Sequence:

Beta waves indicate tense wakefulness.

Alpha waves indicate falling asleep.

Theta waves are slower than Alpha. Still in Stage 1 sleep.

Sometimes a "hypnic jerk" as tonus decreases.

Alpha and theta waves are background, punctuated by short K-complexes and sleep spindles that last for one-half to a full second. If the Alpha and theta waves are the continuo, K and spindles are drumbeats or trumpet notes.

Delta wave is yet slower, and heralds deep sleep, called Stage 4 sleep. K-complexes and spindles disappear.

This deep sleep is replaced by shallow sleep- and then...

REM sleep appears. This stage was noticed by Kleitman and Aserinsky in 1953. It was missed for so long because it only appears some ninety minutes after falling asleep, and then last only for five to ten minutes. It was first noticed in infants who were being investigated because they conveniently sleep during the day, and so much of their sleep is REM.

When adults are awakened from REM sleep, they report dreams.

REM is characterized by Stage 1 brain waves, rapid eye movement and a complete loss of muscle tonus. This muscular paralysis affects everything except the eye muscles and the penis. [And the bladder?]

Muscular paralysis prevents transmission of nerve impulses from motor cerebral cortex to the spinal column.

Eye muscles continue to work because they aren't controlled via spinal column, but through special nerve fibers protruding from the brain stem.

People who enter REM sleep immediately, without passing through the other stages, are suffering from narcolepsy.

In REM sleep, the respiratory and pulse rates undergo radical fluctuations, as though the sleeper were in the throes of a strong emotional upset.



...results of the simple fact that the earth rotates about its axis every 24 hours.

Periodicity is affected by our environment, our internal clock, and integration between the two.


Sleep Gates: times during the 24 hours day when we are internally programed to go to sleep more easily than at other times.

If the Earth didn't rotate, we'd perhaps have two animal kingdoms- Creatures of Light and Creatures of Dark.

Infants' sleep cycles not an integer of 24 hours, and so naturally drift if there are no environmental constraints.

When all environmental cues (variations in light, temperature, humidity, noise, and also control for relative lack of activity) are removed...

Sleep/wake rhythm drifts from 24 hours to what is called the "circadian" rhythm (meaning " about one day"); it varies from ~25 to 28 hours.

Sleep/wakefulness rhythm in "free time" environments does not change but the periodicity lengthens.

It is assumed that this lengthening is an expression of the biological clock that controls sleep and wakefulness.

Subjects in isolation for months at a time find their sense of time altered: They greatly underestimated the duration of their participation, and this underestimation continued to grow as the periodicity of sleep/wake cycle continued to lengthen.

However, there was no change in the accuracy of their estimation of short periods of time.

Are we observing the biological clock at work? Yes. The source of the sleep/wakefulness cycle is the nervous system and not the environment.

What synchronizes the "sun" clock with the "body" clock? Research just beginning to suggest some answers.

In 1996 Clifford Saper at Harvard discovered that when a tiny cluster of nerve cells in the anterior hypothalamus is turned on, all brain cells involved in arousal are shut down. Conversely, when the switch is turned off, the brain wakes up. This mechanical.

The consolidation of sleep/wake rhythm in infants shows that in early months a biological clock is in control, superseded later by environmental cues.

The sleep stages cycle through the night. REM sleep lasts longer and its dreams become more detailed with each succeeding cycle.

A young person has four or five sleep cycles a night while an older person has fewer.

Lots of REM sleep in fetuses and infancy. Stabilizes at about 25-30% of total sleep in adulthood.

As a person ages, total amount of deep sleep decreases. But proportion of REM sleep to total remains constant, at ~20-25%.

Sleep in the elderly is interrupted sleep.

Dreams, thought fragments, and "hypnagogic hallucinations."

Dreams occur during REM sleep, and become progressively more detailed in later REM stages. The spontaneously remembered dream is usually the last one of the night, and is probably exceptional in comparison with those dreamed early in the night.

..although the story of a dream is stored in a readily accessible memory bank, it remains there for a very short time. Any diversion of attention and the memory banks close.

In 80 percent of awakenings during REM, dreams are reported, as compared to ~7% of awakenings in other sleep stages.

Only thought fragments seem to cross people's minds during non-REM sleep.

Higher cortical arousal during REM may be responsible for increased detail and plot of REM dreams.

REM dreams are a "single-minded process" that external stimuli at the time of dreaming do not affect.

The hypnagogic hallucinations of transitional sleep are associative, sometimes pictorial experiences, that can be altered by external stimuli.

Lucid Dreaming: People can remain self-aware during REM sleep.

Lucid dreamers can perform complex tasks according to directions given to them prior to falling asleep.

We appear to be able to control our memory of dreams.

In study of Holocaust survivors: REM dream recall varied from 33% to 55%, as compared with 78% recall of control group of native-born Israelis.

Surprise was that the "well-adjusted" survivors have low incidence of recall and the few dreams that were recalled were devoid of any display of feelings. "The survivors themselves displayed almost total indifference to them after awakening.

[Lavie does not define "well-adjusted" in any detail.]


We track dream objects with our eyes, and single isolated eye movement also appears associated with memory/data retrieval "from the brain's data storage in order to build the dream story."

Animal experiments indicate a relationship between dream sleep, the processing of information and the consolidation of traces of memory in the brain.

Only sleep differences between the eye movement of pre-modern tribes and Westerners was in eye movement, which was much less for the tribes. This may reflect that tribes have less information to process.

HOWEVER, in humans this is not totally understood. Francis Crick hypothesized memory consolidation during sleep, and particularly REM, at the same time that Lavie presented his patient, Mr. Y., who did without REM sleep altogether, with no apparent adverse effects.

REM dreaming may simply be a wonderful but totally unnecessary artifact of the transition from sleep to full wakefulness. Dolphins and Lavie's patient, Mr. Y. seem to argue against the absolute necessity of REM sleep for neural integration and maintenance in adults.

Do fish sleep?

Fish sleep is sleep-like as characterized by Lavie: behavioral quiescence, a typical species-specific posture, elevated arousal threshold, and rapid change in state after intense stimulation.

All of the above, together with characteristic variation in the electrical activity of the brain, probably first appeared in the forebrain of warm-blooded animals.

REM stage in chickens accounts for no more than 3-12% of total sleep, compared to ~25%-30% in adult humans.

Birds have no loss of muscle control in what little REM sleep they do have.

The larger the animal the shorter the sleep.

Majority of large animals sleep on their feet, lying down only for REM sleep.

Length of sleep could be inversely related to metabolism as well as to need to forage for food.

REM sleep varies with the degree of maturation at birth.


Dolphins and echidna are the only two mammals studied so far in which REM sleep is absent or different from that of all other mammals.

Dolphin sleeps with only half its brain at a time and REM stage appears to be absent. The two hemispheres alternate every one to three hours during sleep.

Prevention of sleep in one hemisphere is compensated in that hemisphere only and doesn't change the sleep pattern of the second hemisphere.

The alternation of hemispheres preserves the dolphin's ability to keep breathing during sleep. Most mammals have two brain centers controlling respiration, but the dolphin has only one, the voluntary center.

[Apparent lack of REM sleep not fully explained.]

Cat's experience lots of REM sleep. Experimenter cut the neural pathways preventing muscle tone during REM and watched cats act out their dreams.

As soon as the cat entered paradoxical [REM] sleep, it stood up and embarked upon the most complex behavioral repertoire, which bore no relationship to what was happening around it.

Note that sleepwalking in people occurs in deep sleep of stages 3 and 4.



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