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Sleep occurs because of the happy
coincidence of two natural forces. These are the accumulation of a sleep debt
or a need to sleep due to a period of prior wakefulness and a daily
physiological urge to sleep that comes as a part of the body's physiological
24-hour rhythm. Our sleep debt starts accumulating from the moment we stop
sleeping and increases the longer we are awake. It is the price we pay for wake
and sleeping pays it off. We usually need about 8 hours of sleep to fuel
wakefulness for the next 16 hours. After 16 hours, it is sufficiently strong to
support sleep onset. Without dissipation by sleep, our sleep debt will increase
in strength but reaches a maximum level after about 24 hours and will dominate
all of life thereafter.
Without sleep we would eventually die.
The drive for sleep is so strong that sleep occurs long before death is a real
consequence. We know that you can kill rats with severe sleep deprivation and
there have been extremely rare cases of fatal insomnia in humans.
Rest from activity on a daily basis is a
fairly universal trait among all animals. We differ from most of the simpler
animals on earth by possessing the ability to regulate our body temperature and
share with all other warm-blooded animals (mammals and birds) a natural state
of unconsciousness on a daily basis that we call sleep. The cold-blooded
creatures rest but the brain structures to support sleep are largely absent.
There is no convincing evidence of sleep with its unique brain activity amongst
reptiles and lesser animals.
The body organizes itself to accomplish
specific tasks at set intervals within the 24-hour day. Darkness is the
preferred time for sleep and rest. Our body prepares for sleep by orchestrating
a complex set of physiological changes that favor wake activity in daytime and
sleep at night in a rhythmic manner. This includes modulation of hormone and
neurotransmitter levels (i.e. body temperature, blood pressure, heart rate, and
cellular events of varied complexity). The net results is physiological support
of the sleep process as our daytime activity ends. Sleep induction is also
favored as our sleep debt reaches a significant level after about 16 hours of
wakefulness. The merging of our sleep debt pressure with physiological support
of sleep results in a strong sleep urge at bedtime.
WHAT IS
NORMAL SLEEP?
Sleep is associated with quiescent behavior, a reduced responsiveness to
stimulation, profound changes in brain electrical patterns and significant
changes in physiology. Two dramatic changes associated with the transition from
wake to sleep are amnesia for thought and a loss of volitional control of
thought processes. Sleep also contains a peculiar phenomenon called dreaming
where there is a strange admixture of real life experience accompanied by
bizarre distortions of time, place and physical limitations. Dreams
spontaneously occur within sleep about every 90 minutes throughout the night in
a paralyzed body but with a very active mind that moves through scenes that are
impossible to predict and often without much of a clue as to "Why
this?".
HOW IS
NORMAL SLEEP ACHIEVED?
The experience of natural sleep requires that one is observing at least four
fundamental habits of good sleep hygiene, which are:
- maintaining a regular evening
bedtime.
- sleeping about 7.5 hours nightly and
remaining awake by day.
- sleeping in a dark, quiet and
comfortable location.
- abstinence from stimulating or sedating
herbs and medicines.
Any exceptions to these assumptions will
preclude normal sleep as we are defining it. Therefore, the onset of natural
sleep is a predictable and pleasant experience, which requires a recumbent
position and a favorable environment and lasts for about eight hours. Lastly,
it is terminated spontaneously without aid and results in a refreshed body and
mind.
HOW DID WE
COME TO UNDERSTAND OUR CURRENT CONCEPTS ON SLEEP?
We knew very little scientifically about sleep before 1950 and little about the
disorders of sleep before 1970. It was the discovery in 1953 of a second state
of sleep uniquely associated with dream reports that marked the beginning of an
explosion of new information about the science of sleep. We now know that
during dreams there is a redirection of the brain's neural firing patterns.
These include occasional bursts of conjugate and rapid eye movements, skeletal
muscle paralysis, brain wave patterns that look close to those of wakefulness
and spontaneous irregularities in autonomic control of breathing and heart
rate. It was the later discovery in the late 1960's that significant breathing
abnormalities in sleep accounted for severe pathological cardiovascular
consequences and profound daytime sleepiness that clinical services for sleep
problems developed. There followed the rise of sleep disorder centers, the
development of a classification system for sleep disorders and the birth of a
new discipline called sleep medicine. This assured further scientific study of
the sleep process.
HOW IS THE
TIMING OF SLEEP CONTROLLED?
The timing of sleep is predictable. It occurs about every 24 hours but changes
in light-dark cycles and other environmental queues influence its timing.
Bright light in the few hours before sleep onset tends to delay the time of
sleep onset and also delay awakening the next morning. Thus, bright light in
the bedroom at bedtime can phase delay sleep within the 24-hour day. On the
other hand, bright light exposure at the end of the normal sleep period tends
to move sleep the other direction (backwards or earlier) in the 24-hour period
or advance the phase of sleep. Thus, third shift workers find their urge to
sleep inhibited by light exposure at the end of their work shift. This same
mechanism of light sensitivity is used by animals to determine their daily
schedule. Essentially, they start their day at sunrise and end their day at
sunset. Therefore, they can adapt to seasonal changes in day length or to time
zone shifts during migration. We can use the same principles to adjust when we
sleep if light exposure is systematically applied.
WHY DO WE
SLEEP IN DARK, QUIET COMFORT?
In every brain there is a core of neurons in the lower brainstem that is
necessary for vital functions such as breathing, sleeping, and wakefulness
referred to as the reticular formation. Destruction of this tissue will result
in a permanent comatose condition. This collection of neurons is referred to as
the waking center of the brain. Stimulation of the waking center arouses from
sleep and prevents sleep onset. Sleep requires that we minimize sensory
stimulation because it drives the waking center of the brain and postpones the
normal drive for sleep onset. For this reason, we seek a quiet, comfortable,
and dark environment and a disengagement from energetic activity or stress.
Stimulation of the waking center of the
brain can come from two general sources. One is intrinsic from our own thought
processes. We do not readily fall asleep when we are worried, angry, tense or
in mental turmoil or excitement for any reason. These kinds of thought
processes are coupled with release of adrenaline via the sympathetic nervous
system. Once released, adrenaline requires about 30 minutes to be metabolized
to remove its alerting effect. This is why a time to relax between activity and
sleep is suggested. For those who are having trouble controlling thoughts,
meditation, pleasant reading, soft slow music and other relaxing techniques may
be recommended. The other source of stimulation to the waking center comes from
the numerous sensory pathways that ascend to the sensory cortex. These pathways
also feed parallel projections to the waking center to improve the cortex's
ability to translate their input. A large part of the brain's cortex is devoted
to processing such sensory information. Pain receptors are widely distributed
all over the body both at the surface and internally. We also are sensitive to
touch, position through stretch receptors and temperature. There are several
large cranial nerves devoted entirely to sensory reception including, cranial
nerve I for smell, II for vision, most of V for sensation to the face and head,
VIII for hearing and balance, and a major portion of X for gut sensations.
Sleep is favored when the sensory cortex and the waking center is not fed
information. Closing the eyes rests the visual cortex, a comfortable
temperature minimizes thermal distress, a relaxed position puts position
receptors in a neutral mode and we minimize sensory input. Our internal thought
processes, the other threat, need to be peaceful to disengage the sympathetic
nervous system and its alerting power.
WHAT
HAPPENS AT SLEEP ONSET?
When preceded by the above precautions, sleep onset is a spontaneous and
pleasant experience. It should not require sedation to achieve. When we lay
down to sleep, brain wave patterns (EEG or electroencephalogram) of wake
reflect a firing of neurons in rapid but asynchronous manner referred to as a
low voltage fast EEG tracing. When we close our eyes, the eye muscles producing
coordinated and focused vision relax and the eyes roll into a relaxed position.
Brain wave patterns over the visual cortex at the back of the head develop a
high degree of synchronous activity called alpha waves. Usually within 10
minutes there follows an increase in synchronous firing of neurons over the
entire brain surface with theta waves (3-7 cps) replacing the waking brain
pattern over the entire cortical surface and sleep onset is judged to have
occurred. If awakened at this moment, most subjects will report that they are
not asleep but in relaxed wakefulness. It requires about 10 minutes of sleep to
lose direction of the thought process and amnesia for wake. Conscious thought
does not stop at sleep onset. Therefore, it is common to believe that you are
awake at sleep onset and to believe you have been awake all night if sleep is
highly fragmented by frequent wake episodes.
Loss of consciousness takes several
minutes to occur after sleep onset and will reduce your responsiveness to all
other senses including hearing. This is why falling asleep in front of the
television can occur even though we think we are in touch with the program. We
first lose visual contact and then the loss of audio contact will occur several
minutes later when we progress deeper into sleep. Remember this the next time
you are driving while drowsy. You can have lots of denial and you can fall
asleep and believe you are still awake and driving for a few minutes. If you do
not arouse, sleep will progress and with disastrous consequences. In summary,
the process of transitioning from wake to unconscious sleep may require about
20 minutes on a typical night.
WHAT
HAPPENS AFTER SLEEP ONSET?
Sleep is not homogeneous. Based upon assignment of brain wave patterns into 5
stages, Figure 1, sleep is seen to have a pattern that repeats in 90-minute
units across the night. During the first hour of sleep, we move through
successive stages of non-dreaming sleep. It begins with stage 1 at sleep onset.
In it there are theta rhythms (3-7 cps) in the cortical tracings. Often within
about 10 minutes a new phenomenon appears called sleep spindles because they
resemble a spindle of thread. They are tightly packed bursts of faster activity
(12-14 cps) and occur about every 10 to 20 seconds against the background of
theta activity. Another unusual activity pattern emerges in parallel with the
sleep spindle and is called a k-complex. It is of high amplitude and typically
shows a sharp rise in negative polarity followed by a positive swing in the
opposite direction. They can occur spontaneously either once or several times
per minute. They can also be induced by noise and are thought to reflect
activity in the auditory pathway. The presence of a sleep spindle or a
k-complex signals the beginning of stage 2 sleep. Some believe that their
appearance marks the approximate time when loss of conscious direction of
thought and amnesia for wakefulness occurs. As sleep progresses, the background
brain wave pattern usually slows to about 2 cps and gains in amplitude. When a
threshold of at least 20% of the tracing reaches a slow high amplitude pattern,
stage 3 sleep is scored and at 50% of the tracing, stage 4 sleep is obtained.
As the 90-minute mark is approached, there is a reversal of this pattern and
stage 4 yields to stage 3 and then back to stage 2 followed by a transition to
dreaming or REM sleep.
REM (rapid eye movement) sleep is
defined by the simultaneous convergence of three rather dramatic changes. These
are the loss of sleep spindles, k-complexes and slow waves with a pattern close
to stage 1 in the EEG, the complete loss of skeletal muscle tone and episodic
bursts of conjugate and rapid eye movements. The first dream episode is rather
brief and may last only about 10 minutes. It is followed by another cycle of
sleep like the first and transitions into another REM sleep episode, which will
be a little longer. Thereafter, the interval between dreams is usually limited
to only stage 1 and 2 sleep and the dreams themselves continue o expand in
length. A typical sleep period will contain four dream episodes that constitute
about 25% of the night's sleep. Stage 1 is usually limited to about 5% of sleep
time, stage 2 is 50% of sleep and stage 3 and 4 combine to make up about 20% of
sleep. These would represent quotas for a healthy young adult.
HOW DOES
SLEEP END?
Sleep usually terminates at the end of the last dream episode. There is a
spontaneous transition to wakefulness associated with the feeling that you are
through sleeping. Your body temperature, heart rate and blood pressure have
been steadily declining toward their daily nadir while you slept. There will be
a small but definite rise in heart rate and blood pressure when you stand up
and a rise in body temperature. Your baseline heart rate, blood pressure and
body temperature will each steadily rise to their peak values by late afternoon
along with most mental and motor function skills. It is these inherent changes
associated with other physiological processes that support wakefulness. It is
why daytime sleep is usually terminated prematurely in third shift workers who
try to sleep against this underlying physiological urge to wake up.
This is the gist of normal sleep. You
may have lots of questions. Please feel free to ask or read other headings that
may answer your questions.
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