Professor Dave again, let’s take a nap.
It seems like common sense that we should go to sleep every night, because we are so
used to it.
But if you think about it, it’s a rather bizarre concept.
Every night, we go and lie down in a room, close our eyes, and the conscious mind shuts
down for seven or eight hours, on average.
Why do we do this?
What purpose does it serve?
And if we are unconscious while asleep, then what’s going on in our brains during this time?
Of course everyone can recall at least a handful of vivid dreams they have had, so what are
those exactly?
Why do we dream, and where does that happen in the brain?
Let’s get a closer look at the biopsychology of sleeping and dreaming now.
To understand sleep, we must first describe the stages of sleep.
There are three stages of sleep, as can be distinguished by brain waves on an EEG, which
we introduced in a previous tutorial, so be sure to review that one if necessary.
In stage one, called N1, a person has shut their eyes and is drifting off to sleep.
Theta waves begin to overtake the high-frequency waves of the alert state, and then once asleep,
a low-voltage and high-frequency signal develops.
As we move through the stages, the voltage will slowly increase, while the frequency
slowly decreases.
Looking at stage two, or N2, there are two important features to the EEG.
We can see K complexes, which always involve a large negative wave followed by a large
positive wave.
We can also see sleep spindles, which are brief bursts of 12 to 14 hertz waves.
Stage three, or N3, involves a transition to delta waves, the largest and slowest waves,
and these correspond to deeper sleep.
As we said, this is accompanied by lower frequency and higher amplitude brain waves.
Once in stage three, a person will remain there for some time, and then eventually go
backwards through the phases back to stage one.
It is after this point that a person will now exhibit REMs, or rapid eye movements.
This is where the eyes dart around rapidly under the eyelids, and the corresponding EEG
will look similar to that of a person who is awake.
One cycle through these stages lasts approximately ninety minutes, and this cycle will repeat
for as long as a person is asleep, although as time passes, more time will be spent in
stage one, and less time will be spent in other stages, especially stage three.
So essentially, we can divide sleep up into REM sleep, and the rest, or non-REM sleep,
while N3 is specifically referred to as deep sleep.
Not surprisingly, along with eye movement, REM sleep is characterized by pronounced cerebral
activity, approaching waking levels.
This means that this is the stage of sleep where dreaming most commonly occurs.
As to precisely why we dream, there are hypotheses in an evolutionary context regarding memory
storage, emotional processing, and other things.
But this is not yet completely clear.
Even less clear is what dreams mean.
Prominent figures such as Sigmund Freud have historically put a lot of weight behind dreams,
insisting that they are intimately correlated with our goals and desires.
While there is not very much evidence to support this, it seems undeniable that subconscious
fears and anxieties crop up in our dreams from time to time.
In contrast, activation-synthesis theory proposes that information supplied to the cortex during
REM sleep is more or less random, and that dreams are simply the results of the cortex
trying to process those random signals.
So in short, most dreams are probably relatively meaningless, perhaps even a dumping ground
for the detritus that we accumulate in our memories every day, while some dreams may
actually have a loose correlation with reality.
But an in-depth overview of dream analysis will have to wait for another time.
Right now let’s quickly point out a few more key items regarding sleep and brain activity.
Let’s return to that fundamental question, why do we sleep?
Most relevant theories focus on recuperation or adaptation.
Recuperation theories propose that wakefulness disrupts homeostasis, and sleeping allows
all the cells in the body to perform crucial tasks without the burden of servicing a thinking
and moving organism.
This would imply that sleepiness is triggered by a deviation from homeostasis, and once
this is recovered, wakefulness resumes.
By contrast, adaptation theories propose that we do not sleep in an attempt to achieve homeostasis,
but rather that we simply have evolved to sleep at night, because night is dark, and
darkness is dangerous.
Thousands of years ago, before humans discovered electricity, nighttime was a very dark place,
making us vulnerable to predators, or even accidents.
In addition, sleep allows for the conservation of resources, namely energy, which is also
beneficial.
This notion of cyclical waking and sleeping correlating with daytime and nighttime is
called a Circadian rhythm, and most species display such behavior, whether diurnal, meaning
awake during the day, or nocturnal, meaning awake during the night.
Test subjects that have no access to clocks, and are exposed to a constant level of illumination,
thus removing any external cues regarding when one should sleep, still tend to sleep
at intervals that approximate the 24-hour cycle, indicating that there is something
innate about this cyclical behavior.
Studies have been done with rats, involving lesions to the suprachiasmatic nucleus of
the medial hypothalamus, which receives input from the retina that communicates cues regarding
day and night.
These lesions disrupt the circadian periodicity of their sleep cycles, indicating that this
timing mechanism is most likely housed in this region of the brain.
In looking at these recuperation and adaptation theories, reality is more likely to lie in
their combination.
Data regarding Circadian rhythms is quite convincing, and even certain hormones display
these rhythms, such as cortisol and melatonin.
It also seems clear that sleep serves an important physiological function in all mammalian species,
even large predators that are not at much risk at night.
Studies have shown that sleep drains the brain’s ventricles of compounds that lead to Alzheimer’s
disease, so these kinds of factors must be taken into account.
In addition, some species sleep as little as two or three hours per day, such as horses,
while the giant sloth sometimes sleeps around twenty hours a day, so it would seem that
some amount of sleep is necessary for survival, while certain species have evolved to sleep
much more for other reasons, as such lengths of time defy the idea that sleep is strictly
for recuperating.
Another extremely important function of sleep is memory consolidation.
Studies have shown that mice prompted to navigate a maze during the day will replay the same
pattern of electrical activity during the night while they sleep, so it seems that sleep
has an important role in the context of memory.
One clear way to study the physiological function of sleep is to study sleep deprivation.
Everyone is familiar with the experience of not getting enough sleep and feeling lethargic
or not quite on point the next day.
This sort of experience is difficult to interpret, because stress is also a huge factor, but
many sleep studies have been done attempting to study sleep deprivation in an empirical
way.
In general, individuals that are subjected to significant sleep deprivation, sleeping
just three or four hours per night, experience an increase in sleepiness, a negative impact
on mood, as well as decreases in basic motor function and responsiveness to stimuli.
When looking at complex cognitive functions, data is not as clear, but many studies demonstrate
deficits in this area as well.
Studies also demonstrate reduced immune function in sleep-deprived individuals, as well as
hormonal and metabolic changes.
However, studies also show that consistently sleep-deprived individuals become more efficient
sleepers, exhibiting a higher proportion of slow-wave sleep.
But there is still much work to be done in this area.
With the basics surrounding sleep covered, let’s move on to the next topic.