Physiology of Sleep
Stages of Sleep
The invention of the Electroencephalogram or EEG has made it possible to monitor the
brain's electrical activity. After all,neurons work through electrical impulses. The EEG
is a machine that picks up and records electrical activity through electrodes attached to
the scalp. The EEG of a waking person is clearly different than that of a sleeping person.
The difference in EEG patterns that occur during sleep has made it possible to divide
sleep into five different stages.
The EEG pattern of our brain during wakefulness is very rapid, irregular and low on
voltage. This means that many neurons are firing at different intervals and at different
times, with different strengths too. It would be like going to a crowded bar and hearing
people taking with each other, some here, some there; it's noisy and you can only hear a
unified hum of voices without clear differentiation between them. This type of irregular
electrical activity in the brain during wakefulness is called beta waves or beta activity.
When we are relaxed, sitting in the couch watching T.V. without any mental arousal, the
EEG records a waveform called alpha waves or alpha activity, slower than beta waves. These
two waves (alpha and beta) represent the irregular firing pattern of large areas of the
brain. Brain activity is desynchronized while in alpha and beta activity.
Sleep is not a random process; instead, it is a very organized one. Sleep has five stages:
stage 1-4 (non-REM sleep) followed by REM sleep. This process is cyclical. Once REM sleep
is achieved, the cycle reverses itself and goes back though stages IV, III, II and again
to III, IV and REM. Throughout the night, this cycle occurs at 90-to-100 minute intervals.
Duration of sleep stages vary across ages.
We experience stage one of sleep when we are half asleep and half awake; our
eyelids feel heavy, we feel groggy and suddenly without notice we fall asleep. Stage one
counts for more or less 5% our total sleep during the night. In stage one, we are relaxed,
breathing is more regular and deep. We less aware of our surroundings, we begin to slowly
open and close our eyelids and begin to roll our eyes.
Stage one is characterized by the presence of theta waves, slower and stronger waves than
alpha and beta waves. Theta activity is more regular and has higher voltage than alpha and
beta waves. This suggest that mental relaxation has started. We close our eyes, and within
5 to 10 minutes, we enter stage 2.
In stage two, we still get theta waves plus K complexes and sleep spindles. Sleep spindles
are sudden and intense bursts of electrical activity that last for a second or two. K
complexes are also sudden and sharp waves of a longer frequency, often but not always
triggered by noises. They naturally occur at a rate of one per minute too.
Some researchers believe that sleep spindles and K complexes help us disconnect from the
outside world (noises, etc). The elderly have fewer sleep spindles and they are easy to
Stage Three and Four or Slow Wave Sleep
After about fifteen minutes, we are ready to enter stage 3. Our bodies and minds are
relaxed. The muscles of our bodies are relaxed. Our heartbeat and respiration are slow and
When we enter Stage 3, our brain activity is synchronized. This means that big groups of
cells are firing at about the same time. Think about the example with people in the bar,
but now instead of talking to each other randomly, they are all singing at the same. Now
voices are unified; there is little noise outside the singing. This unified activity in
the brain is represented in the EEG recordings by large waves called delta waves. Delta
waves are the slowest and strongest waves that our brain produces. Stage 3 has about
20-50% of delta waves. Stage 4 is all delta activity. Stage 3 and 4 are also called slow
wave sleep because of the appearance of delta waves.
A young adult spends about 8% of sleep time in stage 3 and 11% in stage 4. It could be
difficult to wake up somebody in slow wave sleep. However, if awakened the person seem
confused and acts slow. During stage 3 and 4, growth hormone is secreted by the pineal
gland. This hormone encourages bone and muscle growth in children, while in adults it is
involved in tissue repair. This is another reason slow wave sleep has restorative effects.
Other hormonal activities, like the secretion of prolactin and gonadotropin occur in these
stages, making slow wave sleep a phase of rest and healing.
REM Sleep (Rapid Eye Movement)
At around ninety minutes after the onset of sleep and following stage 4, the synchrony of
brain waves is suddenly interrupted by desynchronized activity, similar to when we are
awake (Alpha and Beta activity). At this point our muscles are very relaxed; in fact, we
are paralyzed. We are mostly disconnected from the outside world, but we might wake up to
something meaningful like our names being called out, or loud sounds.
In this stage, the electrooculogram or EOG (a device that uses electrodes to pick up
muscular activity of the eyes) senses that the eyes of the sleeper are moving rapidly. At
the same time, the electromyogram or EMG (picks up muscle activity) senses no activity in
the rest of muscles: the patient is paralyzed. The reason we might loose muscle tone is so
that we do not act out our dreams, to keeps us away from trouble.
A single REM cycle lasts about 20 to 30 minutes in a healthy person in his/her twenties.
But as we will see, this period is very prolonged in newborn and children and is
drastically shortened in the elder. What is common to all ages is the cycles of REM
sleep-Non REM sleep.
REM sleep occurs every ninety minute intervals after we fall asleep. This means that in an
8 hour sleep we will experience four or five REM sleep stages and every time we enter REM,
we stay a bit longer in it.
When sleepers in REM stage are awaken, usually they seem alert and can react normally. If
the sleeper was awaken during slow wave sleep, it takes a few instants to wake up and
react. This might be due to the presence of alpha and beta waves in REM sleep and delta
activity in slow wave sleep. Remember that alpha and beta are brain waves are also present
when we are awake, not only in REM. This is why some researchers believe that REM sleep is
an evolutionary adaptation to wake up and be fully alert to a potential emergency in the
middle of sleep.
Other characteristics of REM:
Blood flow to the brain is increased
Genital activity (in males, penis erection and in females, vaginal secretions, these not
necessarily indicate that the sleeper is experiencing a dream of sexual content).
One of the most fascinating aspects of REM sleep is the fact that we experience dreams
during this stage. Sleepers when awaken during this stage report a that they were
dreaming. People woken up during stage 3 or 4 and asked what were they dreaming report
they had ideas or emotions but no narrative dreamlike experiences like in REM sleep.
How are REM and dreaming related?
Comparing the eye movements while awake to the eye movements while in REM sleep, show
almost no difference. It is thought that eye movements during REM sleep are related to the
scanning of the visual scene of a dream. In other words, we are looking at the
surroundings in the dream the same way we do to the surroundings when we are awake.
Another interesting fact is that brain areas in charge of controlling and planning our
body movement and areas that receive sensory information (motor and somatosensory cortex)
are active during REM sleep. We have the vivid sensation of moving and feeling while
dreaming. This means that we actually feel the dream as though it was real. (Go to REM and its traditional association to dreaming)