18. Sleep & Dreaming

12 01neurLecture 18; November 14, 2013Sleep & dreaming

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Introducing sleep After finishing this course, youll have a reasonable understanding

of some of the most famous motivated behaviors, sex and eating being the two major ones.

As esteemed as these behaviors are, we spend a great deal more time sleeping than we do engaging in either.

Most people sleep over 175,000 hours in their lifetimes. This can vary a great deal between people, depending on their sleep

habits. It can be quite depressing to think of how many hours wed save if

only we could sleep less.

The fact that we spend so much time sleeping suggests that it must be good for something But what is sleep actually good for?


Introducing sleep The amount of hours per day an animal

spends sleeping has no clear relationship with any aspect of its physiology or anatomy. Contrast this with things like food intake,

metabolic rate, water intake, etc., that scale with the size of the animal.

Donkeys may only sleep 2-3 hours per day. Opossums over 18h. Humans fall near the lower end of the spectrum, sleeping an average of 6-9 hours per night.

Non-terrestrial mammals (whales, seals, walruses etc.,) have unusual sleep patterns. In certain cases, their version of sleep

happens one hemisphere at a time.


Stages of sleep Early researchers studying the brain

during sleep made two interesting discoveries.

1. The brain is quite active during sleep, at certain times showing levels of electrical activity that are equivalent to being awake. Prior to this discovery, it had been

assumed that the brain went into a state of dormancy or inactivity during sleep.

2. The brain goes through several discrete stages while sleeping. Sleep is heterogeneous with respect to the brain. Prior to this discovery, sleep was

considered to be a unitary, on/off function.

A participant in a sleep study pictured wearing EEG electrodes.


Stages of sleep Using a machine called an

electroencephalogram (EEG), it is possible to record the brains electrical activity using electrodes placed on the skull.

When a lot of neurons are firing separately, one sees a low-amplitude, high-frequency pattern of EEG waves.

When neurons are firing in a synchronous manner, the frequency of the EEG wave goes down, but the amplitude goes up.

Amplitude

Frequency (hz)

High Low


Stages of sleep When the individual is awake and fully

alert, the EEG shows beta waves. Beta waves are high-frequency, low-

amplitude waves.

As the individual begins to get drowsy, the EEG shows alpha waves. Alpha waves have a lower frequency

and higher amplitude than beta waves. Be careful not to mix up the order of

these waves (you might be tempted to think that alpha waves come before beta waves, but thats not true).

During deep sleep, the individual shows delta waves on the EEG. Delta waves are characteristic of non-

REM sleep, and have an extremely low frequency and high amplitude.

Illustration from Kolb & Wishaw, An Introduction to Brain and Behavior. Sinauer, 2014


Non-REM sleep Non-REM sleep, also called deep sleep, is a

time during which a large range of activities take place. For example: body temperature decreases,

growth hormone secretion increases.

Dreaming does occur during non-REM sleep, but the dreams are reported to be much less vivid than those seen in REM sleep.

It is during non-REM sleep that people exhibit sleep-talking (somniloquy) and sleep-walking (sonambulism).

It is also during non-REM sleep that people can experience night terrors. Night terrors are brief, extremely frightening

dreams. They are often experienced by children.

People are free to move during non-REM sleep. This can lead to much tossing and turning.


REM sleep Several times each night, the brain undergoes an

interesting shift from the slow wave sleep (characterized by delta waves) to a state known as rapid eye movement sleep (REM sleep)

REM sleep can easily be identified in people by observing their eyes. During REM sleep, the eyes move rapidly in every which way.

During REM sleep, the brain shows a pattern of EEG waves that closely resembles the beta waves of the awake state. This suggests that the cortex is somewhat awake during

REM sleep.

Most of our vivid dreams take place during REM sleep. We do not typically remember our dreams, but people who

are woken up in the middle of REM sleep almost always report having been in the midst of a dream (appx. 80% of the time).

The muscles are paralyzed during REM sleep. This is presumably to stop us from acting out our dreams. This also means that instances of sleepwalking and sleep

talking cannot take place during REM sleep.

Even animals that mostly stand while they sleep (horses for example) must lie down for REM sleep.


The stages of sleep A nights sleep can be divided into sleep cycles

that last about 90 minutes each.

In every sleep cycle, the individual passes through each of the stages of sleep, finally ending in a period of REM sleep. The cycle continually repeats itself until the

individual wakes up.

Your success in being able to wake up depends on which stage of the sleep cycle you happen to awaken during. Left to your own devices, your brain tends to wake

you up only after youve entered a very light stage of sleep, and its not so hard to get up.

If your wakeup time is determined by an uncaring alarm clock, then you could easily get woken up during slow wave sleep. The resulting grogginess is called sleep inertia.

As the night progresses, the sleep cycle becomes increasingly dominated by REM sleep.

The amount of time spent in REM sleep varies over the course of the night.



The stages of sleep

For unknown reasons, the percentage of time people spend in REM sleep gradually declines into adulthood, as does amount of sleep overall.



Why do we dream? The fact that we need to sleep is strange

enough, but the fact that we spend a portion of the time vividly hallucinating, only to experience amnesia about the event upon waking up is especially strange.

It is unlikely that dreams are essential to

mental functioning. Certain antidepressants almost completely

eliminate REM sleep (and presumably dreams as well), yet people on those drugs have no obvious deficits.

There is no evidence that the content of dreams has any particular hidden meaning. Dream interpretation is largely a

pseudoscientific enterprise. See side panel

Two separate dream interpretation websites give two entirely different interpretation of pandas! Ambiguous stimuli apparently dont have any consistent meaning, and theres no use in reading too far into them.


Dreaming: the activation synthesis theory During waking life, the brain spends much of its time on

taking in sensory information from the sense organs, and converting that information to a useful form. The cortex interprets incoming sensory information. It

organizes it according to previous experiences.

During sleep, the brain does not receive much in the way of sensory stimulation. In particular, the brain is not provided with any visual

information during sleep. Auditory and olfactory information from the outside world can occasionally make it into dreams.

Random information is produced by the brainstem and sensory regions, and according to the activation synthesis theory, dreams result when the cortex attempts to interpret this information.

Because the cortex uses past experiences and memories to make sense of these signals, dreams have a vague relationship with our waking lives.


Dreaming: an evolutionary theory Negative imagery is a very common theme

in dreams. Our dreams often center around dealing with threatening situations.

Antti Revonsuos evolutionary theory proposes that dreams may serve an adaptive evolutionary purpose by acting as training simulators, allowing us to practice dealing with real-life threats in a safe environment.

This could explain why the fears expressed during dreams are often very primitive (the sensation of being chased, in danger, etc.,) and why they often incorporate the dreamers ongoing emotional problems.


How much should we sleep? Sleep deprivation is not fatal in humans.

People have successfully deprived themselves of sleep for weeks at a time. This is not a very pleasant process.

Drowsiness becomes overwhelming, and people engage in short microsleeps whenever they get a chance.

Sleep reduction or deprivation most notably aects executive function in the brain. Numerous other effects are seen throughout

the body including increases in blood pressure, decreases in immune function, and changes in metabolism.

People who slept between 5-7 hours had the lowest rates of all-cause mortality over a 10 year study period. This study (Tamakoshi & Ohno 2004)

controlled for almost every possible confounding variable, but since it is correlational, it does not prove that reducing sleep can reduce mortality.

People who slept 7 hours a night have the lowest rates of mortality (within the period of study we all have a 100% rate of mortality eventually).


Sleep disorders The disorders of sleep can broadly be

divided into dyssomnias and parasomnias.

Dyssomnias involve diculties getting enough sleep, problems with sleeping at appropriate times, and reductions in the quality of sleep. E.g. narcolepsy, insomnia, hypersomnia

Parasomnias involve abnormal behavior or physiological events that occur during sleep. E.g. nightmare disorder, sleepwalking disorder


Dyssomnia: Insomnia Almost 33% of the population will report some symptoms

of insomnia during a given year. Women report insomnia twice as often as men.

Insomnia can be associated with disorders such as depression, anxiety disorders, substance use, and dementia (though hypersomnia can also be associated with all of those).

Insomnia is often treated with sedative-hypnotic drugs such as Ambien and Xanax, both GABA receptor agonists. These drugs can be quite effective, its not practical to take

them forever, and they tend to cause rebound insomnia when the individual quits.

Insomnia can also be treated with melatonin, a hormone involve in regulating circadian rhythms. Melatonin secretion normally peaks at night, and nighttime

supplementation of melatonin amplifies this effect. However, there is only mixed evidence that it actually helps with sleep in humans.


Parasomnias Parasomnias can occur during both REM and non-

REM sleep, though each stage produces its own symptoms.

Parasbomnias that occur during non-REM sleep typically involve odd or unexpected movements. Sleepwalking (sonambulism) is an example of this.

Nightmares occur during REM sleep.

Sleep paralysis is a phenomenon that can occur during REM sleep. During an attack of sleep paralysis, the individual partly

awakens, but remains paralyzed due to REM sleeps suppression of muscle tone.

This is often accompanied by an overwhelming feeling of dread, and the sense of the monstrous presence in the room.

Sleep paralysis is often accompanied by a feeling of pressure on the chest. Many cultures have ascribed this to the activity of evil spirits.


Sleep and memory Early investigations into the issue of

sleep and memory suggested that memories become stronger following a nights sleep. Everyday experience attests to the same

fact.

Karni & Sagi (1994) showed that a normal amount of REM sleep could improve performance on an object discrimination task. When participants were selectively

deprived of REM sleep (by waking them up whenever their brains entered REM) their performance on the task was reduced.


Sleep and memory Given that the individual is not exposed to much

stimulation during sleep, maybe the only reason sleep helps memories is that it provides a passive, quiet time free of distracting stimuli. Alternately, maybe sleep actually helps solidify memories,

protecting them from interference.

To test this, researchers developed a two-stage experiment. In the first stage, participants memorized word pairs (e.g.

blanket-window). In the second stage, participants were made to learn

distracting word pairs (e.g. blanket-sneaker). Participants who were allowed to sleep between the two

stages remembered more of the first sets of word pairs. This suggests that sleep protects memories from interference.


Sleep and memory Emerging evidence suggests that something happens during sleep

to selectively promote certain memories, and dismiss others.

Boring, unemotional memories tend to decay after a night of sleep. But emotionally charged memories actually become stronger after a night of sleep. This suggests that the brain is somehow able to filter through the

days events, and select out the ones that are most important to remember.

Could this be the reason dreams are often emotionally charged, and concern recent events?

This finding also has implications for dealing with post-traumatic stress disorder (PTSD). In fact, it might be worth recommending that people who have just

gone through a traumatic event be subjected to sleep deprivation, to prevent traumatic memories from being stamped in.


Sleep and memory The brain may rehearse certain

memories during sleep.

In one study, rats were trained to search for food in a maze-like environment. While this was happening, researchers monitored the activity of neurons in the hippocampus.

The hippocampus contains neurons known as place cells that are involved in spatial navigation.


Sleep and memory

Interestingly, the same pattern of neurons that were activated by the

maze were also active during while the animal was asleep (during non-REM sleep). This suggests that the animal was practicing the maze while asleep.



Nocturnal practice Musicians have long known that a night of sleep

following a period of hard practice can work like magic. Passages that the individual once struggled with become much easier the next day. The same appears to be true for athletes, and may

generalize to all forms of complicated motor learning.

Researchers explored neuroscience of this phenomenon by having people learn to type complicated number sequences on a keyboard.

After a nights sleep, participants improved their performance on this task.

The degree of improvement was even greater for number sequences that the individual had diculty with the day before.


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Nocturnal practice

Functional MRI brain scanning showed that participants recruited dierent brain regions to complete the task before and after sleep.

Before sleep!

After sleep!


So why do we sleep? Sleep is not a passive process, it is a period of

intense neural activity. The brain clearly invests a lot of eort into sleeping.

While emerging evidence suggests that sleep is important in consolidating memories and clearing the brain of accumulated waste products, some major questions remain to be answered.

Why do so many animals have such dierent sleep patterns? Youd think that if sleep were an essential

homeostatic process, animals would sleep in similar ways.

Why is sleep deprivation not more dangerous? Also, certain antidepressants specifically block REM

sleep. People can take these for years with apparently no ill effects.

Do the beneficial eects of sleep have to happen during sleep, or is it just a coincidence?

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18. Sleep & Dreaming