The Brain During Sleep
Jump to the following topics:
- The stages of sleep.
- REM sleep.
- Non-REM dreams.
- The brain during a
information-processing function of dreams.
The stages of sleep. While we
sleep, we go through a series of stages which differ in their
brain-wave patterns and physiological conditions.
- When we first enter sleep, our brain waves decelerate from
beta (12 to 18 cycles/second) to alpha (8 to 12 cycles/second) to
theta (4 to 8 cycles/second). At this point, stage one begins.
- The stages.
The cycle continues. We continue a 90-minute cycle throughout
our sleep period. Some researchers have speculated that a similar
90-minute cycle persists during wakefulness, with corresponding
variations in brain activity and attentiveness; during the part of
this wakeful cycle which corresponds to REM, we are in a
less-attentive, daydreamy mood, as the brain seems to go off-line
to process our new data. These two types of functioning
(correlating to non-REM and REM) are similar to the analytic
operations of the brain's left hemisphere and the creative reverie
of the right hemisphere.
- Stage one. This stage lasts for only a few minutes of light
sleep. We experience a lowering of body temperature and blood
pressure, relaxation of our muscles, and a slowing of our
breathing and heart rate. Brain waves are slow and low-voltage.
The first "stage one" of the sleep period is called "descending
- Stage two. We are now in a deeper sleep, and more difficult
to awaken. Our muscles relax. Stage two is recognizable by its
predominance of theta waves and the appearance of "sleep
spindles" (momentary occurrences of high-voltage brain waves at
12 to 14 cycles per second) which occur only during this stage.
- Stage three. During stage three, we become more relaxed,
and we experience a decrease in heart rate, body temperature,
blood pressure, and respiratory rate. Our brain waves are
increasingly within the delta range (1 to 3 cycles per second).
- Stage four. We spend about 20 minutes on this level, which
is characterized by profound muscle relaxation, difficulty in
awakening, and a majority of our brain waves within delta. When
we leave stage four, we ascend through stage three and two and
one. As the cycle continues throughout the sleep-period, stage
four becomes shorter -- and by the end of the night, we are
alternating only between stage two and stage one. Stage four is
experienced only two or three times during our sleep period.
- Ascending stage one. (Also called "emergent stage one.") We
return to stage one approximately 90 minutes after entering
sleep. Unlike the first stage one, this is a period of rapid
eye movements and REM dreaming. (Non-REM dreams can occur at
any time during sleep.) The first time we enter "emergent stage
one," the period continues for only about five to ten minutes
before we descend again to stage two and so on; by the end of
the night, stage one might continue for as long as one hour --
for a total of approximately 90 minutes of REM throughout the
sleep-period. REM sleep consists of two stages; "phasic" is the
time of muscle twitching, quick eye movements, fast heart rate,
and lucid dreams -- and "tonic" is a quieter interval.
- REM deprivation. In lab experiments, subjects have been
awakened at the beginning of each REM period, thereby depriving
them of REM sleep (although they were permitted to have non-REM
sleep). After a few nights, some of the people exhibited
psychological disturbances such as irritability, anxiety,
disorientation, paranoia, inability to concentrate, acute hunger,
depression, and decreased motor skills. (However, other people had
virtually no adverse reactions to REM deprivation.) Animals in
similar REM-deprivation experiments have become hypersexual;
others have died. During experiments, some human subjects
responded in these ways:
- Some people would re-enter REM quickly after returning to
sleep; even if they were awakened dozens of times, they would
still attempt to achieve REM sleep. One researcher found that
if the subject remained awake for three minutes after being
disturbed from REM sleep, that period would be skipped, and the
four-step sleep cycle would start at the beginning. In another
experiment, the standard 90-minute sleep cycle became shorter,
as if to return the sleeper to emergent stage one -- where REM
dreaming occurs -- as quickly as possible.
- Their non-REM dreams became weirder, as though trying to
behave like REM dreams. Non-REM dreams are usually similar to
plain "thinking," while REM dreams are strange, story-like
- They hallucinated during wakefulness. This might be another
way in which the demand for "dreams" imposes itself (in the
form of hallucinations) onto time which is normally not set
aside for dreams.
- Their sleep became lighter, as though they were trying to
remain close to stage one where REM dreams occur.
- When the experiments ended, they would spend nearly all of
their sleep-time in REM. This was apparently an attempt to
compensate for the deficit.
Non-REM dreams. Although dreams are
usually associated with REM sleep, the correlation is not exact;
dreams do occur during non-REM periods, and people who are awakened
from REM sleep frequently do not report dreams. Some non-REM dreams
are similar to REM dreams (and vice versa), but they usually exhibit
these differences: non-REM dreams tend to resemble wakeful thinking
(perhaps pondering a wakeful event, or a REM dream which has
occurred), and they are generally less emotional, outlandish,
lengthy, dramatic, visual, and active. When people are awakened from
non-REM dreams, they might say that they weren't asleep at all, but
were awake and thinking; this is the error made by some people who
claim to be insomniacs, although sleep-lab equipment proves that they
were sleeping. In one occasion when I recalled the non-REM state, I
noted that "the thoughts were the same as ordinary daytime thoughts,
and they concerned regular subjects. It was just an ordinary 'mulling
The brain during a dream.
- During dreamless sleep, the brain operates on a subdued level
-- just enough to maintain its basic functions and the body. But
during a dream, it becomes more active. During REM sleep, our
brain has a higher temperature and increased blood flow. The brain
waves are irregular with extreme peaks and troughs; they can be
described as mixed frequency waves, with low amplitude, and only
slight alpha activity.
- Part of the brain might consider our actions to be "real."
When we dream of performing an activity (e.g., running), the brain
sends the same signals that it would send if we were awake and
running. To that part of the brain and nervous system, we
are running; hence, the dreamed experience seems authentic.
However, during REM sleep, these nerve impulses are obstructed
before they reach the muscles, through a function called "sleep
paralysis." This paralysis keeps us from acting out our dreams
while asleep. (In an experiment, cats were given a drug which
counteracted sleep paralysis; during dreams, they ran, pounced,
and hissed in apparent interactions with feline-dream characters
-- perhaps mice or other cats.)
- Our sensory perceptions might seem to be "real." During
wakefulness, our imagination is active, adding emotion and fantasy
to everything which we perceive. However, these fancies are
usually overpowered by sensory input, and they are suppressed by
serotonergic neurons. During REM sleep, we receive no sensory
input, and those neurons are constrained. Thus our imagination is
free to express itself -- and it does so through the same neural
network which would be feeding sensory input to the brain if we
were awake. If we see, for example, a tree during a dream, the
neurons react in the same manner as if they were seeing a
wakeful-world tree. For their purposes, the dreamed tree is
- EEG readings during REM sleep. When a person is awake and
attentive, brain waves are spiked and short. Tiredness causes the
brain waves to become slow and long; the REM period puts the waves
back into a short pattern -- mixed frequency, low amplitude. EEG
readings become more animated as the person shifts from deep sleep
to dreaming. During the first half-minute of REM, alpha waves
decrease in the left parietal region of the brain; as when we are
awake, alpha rhythms are less prominent during vigorous periods
than during calm periods, so this reduction in alpha indicates an
enlivening of the brain. While the dream is occurring, the level
of EEG activity varies according to the amount of commotion in the
dream; during energetic dreams, the brain is more active than
during wakefulness, and the EEG reading is virtually
indistinguishable from that of wakefulness. Alpha rhythms are more
conspicuous when the dream scenario is passive than when it is
- The brain during REM sleep. REM begins when the GTF
(gigantocellular tegmental field) neurons in the pons (bridge) of
the brain stem are stimulated. These GTF nerve cells excite the
lower brain (the seat of emotions) and the cortex (where sensory
data -- including visual data -- is processed). During REM, the
brain emits neurotransmitter chemicals: large amounts of
acetylcholine (which stimulates the cortex), and small amounts of
serotonin and norepinephrine. At the end of the REM period,
dreaming ceases when the locus coeruleus (another cluster of cells
in the brain stem) emits another neurotransmitter, norepinephrine,
to deactivate the GFT cells. In an experiment with a cat, the EEG
readings which correspond to REM sleep were induced by a drug
which is similar to the acetylcholine; those readings returned to
a non-REM status when the cat was given a dose of norepinephrine.
information-processing function of dreams.
- While we sleep, one of the brain's tasks is to process
information which was acquired while we were awake.
- Dreaming is "off-line processing." When a computer has to
catch up on a backlog of data, we make it go "off-line"; we
disconnect it from sources of additional data until the backlog is
consumed. Sleep offers us a similar situation; this is a time when
we are not gaining new experiences, physical challenges, or
sensory input. Another advantage is that the unconscious mind can
review our wakeful life without the interference of the conscious
mind's rigid style of reasoning, limited creativity, habitual
patterns of thought, and restricted access to psychological and
spiritual resources which are available to the unconscious mind.
(However, during subsequent wakefulness, the conscious mind's
logical and intuitive faculties must decide whether the
unconscious mind's proposed solutions are likely to be appropriate
and effective in the wakeful world.)
- Memories are processed during sleep. Many studies have shown
that sleep (and dreams in particular) are necessary for the
workings of our memory. Consider these experiments, from which we
might make the following conclusions:
We spend more time dreaming when confronted with problems. We
process not only memories but also speculations on
problem-solving. We dream more, and sleep longer (probably so that
we can dream more), during phases when we encounter stress or
emotional disturbance from situations such as a new job, or family
turmoil, or a school examination.
We consider and rehearse alternatives during dreams. Nietzsche
said that dreams are a training ground for life. During dreams, we
frequently mock up circumstances which have disturbed us in
wakefulness: we see (perhaps symbolically) those perplexing people
and circumstances. A dream character is created to represent a
troublesome individual, and then our dream persona experiments
with different behaviors which might resolve our distress. If this
process is successful, we discover a solution and we might also
relieve some of the stress which was related to the problem. (For
example, pregnant women who dream about their delivery tend to
have an easier delivery, probably because of their sleepful
A dream speculation might be recognized when we are awake.
During wakefulness, if the event which transpires resembles one of
our previous speculative dreams, we might call the dream
"precognitive," and the event might be accompanied by a deja vu
feeling (because it did happen before -- in a dream). However, in
some supposedly precognitive dreams, we are merely speculating, as
indicated by the "death dreams" of ill people who recover.
- REM sleep is better than non-REM sleep for recall. Subjects
were awakened from sleep and asked to remember some material.
When they awoke again, they were re-tested. The people who had
exhibited more REM sleep remembered more. (This effect is more
noticeable when the material is an abstract mental skill than
it is when the subjects engaged in simple memorization.) In a
different experiment, some students were awakened during
various stages of sleep; if awakened from non-REM sleep, they
continued to progress in a skill which was being studied during
wakefulness -- but if they were awakened from REM sleep, they
did not progress.
- REM sleep is better than wakefulness for recall. Subjects
were asked to remember some information, and they were retested
later; during the intervening period, some of the people slept
and the others stayed awake. If the sleep period was two hours
or less, the sleepers and wakefuls lost the same amount of
recall. But if the sleep period was more than two hours, the
sleepers remembered the same amount as if they had been
awakened after two hours; however, the wakefuls continued to
forget more of the data as time went on.
- REM sleep is more effective for recall of emotional
material. Subjects were tested on their skills in concluding a
story and a crossword puzzle; after REM sleep, they showed a
greater increase in their abilities with the story than with
the crossword puzzle. (A story deals with the emotions of the
right hemisphere; a crossword puzzle involves the logic of the
- REM sleep increases during periods of learning. When rats
were learning to run a maze, they had more REM sleep than they
had had before starting their lessons. When infant humans are
asleep, about 50% of their time is spent in REM (probably to
process the profusion of data from their new world); adults
spend only 20% to 25% of their time in REM.
- Dreaming also affects our types of wakeful memories. In an
experiment, people were tested on their ability to remember a
list of words. Some of the people were deprived of dreams
during the sleep period preceding the experiment; they
remembered more words pertaining to emotions and introspective
themes. The people who had dreamed normally remembered more
words relating to social interplay and personal accomplishment;
we might conclude that they had worked out their inner needs
during dreams and were now ready for the challenges of the