Different Stages of Sleep
Introduction To Sleep
Disorders
Different Stages of
Sleep
Classification and Description of Sleep Disorders
How
Does The Use of Sleeping Pills Affect Sleep
In 1952, sleep-researcher
Nathaniel Kleitman discovered that although sleepers tended to have slow,
rolling eye movements beneath their lids as they fell asleep, during some
portions of their sleep their eyes darted rapidly in a highly coordinated way,
moving more quickly and sharply than they could while they were awake. He dubbed
the phenomenon rapid eye movement (REM), a phase of sleep that was later related
to dreaming.
Later researchers using
electroencephalogram (EEG) that measures the electrical activity in the brain
discovered that the REM stage of sleep is different from non-REM (NREM) sleep.
Sleep occurs in a series of
cycles, each lasting between sixty and ninety minutes. A normal sleep pattern
involves four to seven such cycles during the course of the night. On average,
people have five or six sleep cycles during a normal nighttime sleep session.
Each cycle has two main parts.
During the first part, our
level of consciousness falls while the level of unconsciousness rises. This part
of the cycle involves changes in heart rate and breathing, and an overall
slowing of brain activity. We do not dream during this phase.
In the second part of the
cycle, however, we do dream. The characteristic sign of this phase of sleep is
rapid eye movement, or REM.
Generally, each ninety-minute
sleep cycle contains a non- REM period (or slow-wave sleep) and the REM period.
On average, each of these two main periods occupies about 50 percent of the
cycle's elapsed time, or about forty-five minutes. However, the balance between
the two periods shifts during the course of the night. During the first
ninety-minute cycle, the REM phase might last only a few minutes. In the final
cycle of the night, REM sleep occupies most of the time, perhaps an hour or
more.
Non-REM sleep actually consists
of four distinct substages, labeled 1 through 4. The stages are defined
according to the types and intensity of brain wave activity as measured by EEG
tracings, and also by the ease with which the sleeping person can be aroused.
Threshold
The initial stage of sleep is
known as the threshold. The process of falling asleep in actuality occurs in
seconds. When you switch over into sleep, your awareness of the outside world
vanishes in seconds.
Relaxation just prior to sleep is characterized by a steady, even alpha rhythm
characterized by a low-voltage fast EEG pattern. Gradually, muscular tension
decreases and bodily functions slow down. The sleeper's mind wanders, and his or
her awareness grows dull.
Stage 1
Non-REM Sleep
After the
sleeper falls asleep, he or she enters into what is known as the Stage 1 of non-
REM sleep.
Stage 1
sleep is the period we "drift off."
Stage 1 NREM
Sleep is characterized by the following:
Breathing
becomes slow and even
The
heartbeat becomes regular
Blood
pressure falls
Brain
temperature decreases
Blood flow
to the brain is reduced
Little or no
body movement.
Generally,
this description of bodily functions characterizes all non-REM sleep.
Stage 1
sleep might be called a kind of twilight time. It's initial appearance lasts up
to ten minutes in most sleepers.
Brain waves
become smaller, slower, and somewhat irregular characterized by a low-voltage
fast EEG.
This portion of sleep is distinguished by drifting thoughts and dreams that move
from the real to the fantastic, along with a kind of floating feeling.
The sleeper
is still easily awakened and might even deny having slept.
Stage 2 NREM
Stage 2 is
an intermediate stage of sleep. It initially lasts about twenty minutes. The
sleeper will gradually descend deeper into sleep, becoming more and more
detached from the outside world and progressively more difficult to awaken.
Stage 2 is
characterized by:
Larger brain
waves and occasional quick bursts of activity.
The sleeper
will not see anything even if the eyes are opened.
A sleeper
can easily be awakened by sounds.
Bodily
functions slow down. Blood pressure, metabolism, secretions, and cardiac
activity decrease.
Stage 3 NREM
Stage 3 is
the beginning of deep sleep, occurring about thirty to forty five minutes after
you first fall asleep.
Stage 3 is
characterized by:
Brain waves
are slow (at the rate of 0.5 to 4 per second) and quite large (five times the
size of waves in Stage 2). These brain waves are known as delta waves.
The sleeper
is far more difficult to awaken as compared to stage 1 or 2 sleep. It takes a
louder noise or an active attempt to wake him or her.
Stage 4 NREM
The deepest
sleep occurs in Stage 4.
Stage 4 is
characterized by:
The brain
waves (called delta brain waves) are quite large, making a slow, jagged pattern
on the EEG.
The sleeper
experiences virtual oblivion. If the sleeper is a sleepwalker or a bed wetter,
those activities will begin in this phase.
Bodily
functions continue to decline to the deepest possible state of physical rest
This first
period of deep sleep is the deepest. The sleeper awakened from deep sleep will
probably be groggy, confused, or disoriented. He or she may experience "sleep
inertia" or "sleep drunkenness," seeming unable to function normally for quite
some time.
After the
first phase of deep sleep ends, the sleeper returns to Stage 2 and then enters
the REM state.
