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We spend about one-third
of our lives asleep. But far from being wasted time, from the moment we slide
into unconsciousness, a whole raft of functions takes place to make sure that
we get optimal benefit from our nightly rest. >Sleep is the time the body can
undergo repair and detoxification. Poor sleep patterns are linkedto poor health - and those who sleep less than six hours a
night have a shorter life expectancy than those who sleep for longer. So sleep
has a profound effect on our mental, emotional and physical wellbeing.
Here, Daily Mail writer Angela Epstein explains how
the body works on when we drift off.
The brain
Sleep may seem to be a passive and dormant state,
but even though activity in the cortex - the surface of the brain - drops by
about 40 per cent while we are in the first phases of sleep, the brain remains
highly active during later stages of the night.
A typical night's sleep comprises five different
sleep cycles, each lasting about 90 minutes. The first four stages of each
cycle are regarded as quiet sleep or non-rapid eye move-ment (NREM). The final
stage is denoted by rapid eye movement (REM).
During the first stage of sleep, brain waves are
small undulations. During stage two these intersperse with electrical signals
called sleep spindles - small bursts of activity lasting a couple of seconds
which keep us in a state of quiet readiness.
As stage two merges into stage three, the brain
waves continue to deepen into large slow waves. The larger and slower the brain
wave, the deeper the sleep. Stage four is reached when 50 per cent of the waves
are slow.
At this point, we are not taxed mentally and 40 per
cent of the usual blood flow to the brain is diverted to the muscles to restore
energy. However, during the REM that follows there is a high level of brain
activity.
This is the stage associated with dreaming and is
triggered by the pons - the part of the brain stem that relays nerve impulses
between the spinal cord and the brain - and neighbouring structures.
The pons sends signals to the thalamus and to the
cerebral cortex, which is responsible for most thought processes. It also sends
signals to turn off motor neurons in the spinal cord, causing temporary
paralysis and so preventing us from acting upon our dreams.
REM sleep is thought to help consolidate memory and
emotion, as at this point in sleep blood flow rises sharply in several brain
areas linked to processing memories and emotional experiences. In areas
involving complex reasoning and language, blood flow declines.
The eyes
Though covered by lids, the movement of the eyes
denotes the different stages of sleep.
On first falling into semi-consciousness, the eyes
roll. But as we move into deeper sleep, rapid eye movement (REM) occurs when
the eyes twitch and dart about.
This REM occurs within about 90 minutes of falling
asleep and recurs about every 90 minutes throughout the night. It denotes a
time when most dreaming is done.
Although brain activity is high at this point, the
muscles of the body are relaxed to a point of virtual paralysis. Some experts
suggest that this is a device to allow the mind to explore the realms of
subconscious without acting upon events occurring in dreams.
Hormones
During our waking hours, the body burns oxygen and
food to provide energy. This is known as a catabolic state, in which more
energy is spent than conserved, using up the body's resources.
This state is dominated by the work of stimulating
hormones such as adrenaline and natural cortisteroids. However, when we sleep
we move into an anabolic state - in which energy conservation, repair and
growth take over. Levels of adrenaline and corticosteroids drop and the body
starts to produce human growth hormone (HGH).
A protein hormone, HGH promotes the growth,
maintenance and repair of muscles and bones by facilitating the use of amino
acids (the essential building blocks of protein). Every tissue in the body is
renewed faster during sleep than at any time when awake.
Melatonin is another hormone produced to help us
sleep. Secreted by the pineal gland deep in the brain, it helps control body
rhythms and sleep-wake cycles.
Levels of melatonin rise as the body temperature
falls, to encourage feelings of sleepiness. The opposite occurs to wake us up.
It is mostly during sleep that the sex hormone
testosterone and the fertility hormones, follicle-stimulating hormone and
leuteinising hormone, are secreted.
Immune system
Research suggests that sleeping more when fighting
infectious illness aids recovery.
This may be due to the immune system's increased
production of certain proteins during sleep, as the levels of certain agents
which fight disease rise during sleep and drop when we are awake.
Getting enough sleep can also help resist
infection, as some studies of healthy young adults have shown that moderate
amounts of sleep deprivation reduce the levels of white blood cells which form
part of the body's defence system.
A cancer killer called TNF - tumour necrosis factor
- also pumps through our veins when we are asleep. Research has shown that
people who stayed up until 3am had one-third fewer cells containing TNF the
next day, and that the effectiveness of those remaining was greatly reduced.
JUST as the world is governed by light and dark, human beings also have an
inbuilt body clock called the circadian rhythm.
Housed in the hypothalamus at the base of the
brain, it causes 24 hour fluctuations in many bodily activities. It also
governs the body's daily alternation between sleep and wakefulness, and works
on a 24-hour cycle to let the body 'know' when sleep is coming.
The circadian rhythm regulates all the processes of
the body, from digestion to cell renewal. All these body rhythms are triggered
by an action of a network of chemical messengers and nerves which are
controlled by the circadian clock.
Ensuring regular periods of sleep at night lets the
bodyclock regulate hormone production, so that you feel wakeful and alert
during the day and ready to enjoy restorative sleep at night.
Body temperature
In the evening, body temperature, along with levels
of wakeful hormones such as adrenaline, start to drop. Some sweating may occur,
as the body is immobile and tries to combat losing heat.
Body temperature continues to fall throughout the
night. By about 5am it has dropped to about one degree centigrade below the
temperature it was in the evening.
At the same time, our metabolic rate drops too.
This is the time of day when you would feel most tired, as the low temperature
coincides with adrenaline at its lowest level.
Low body temperatures increase your likelihood of
sleeping deeply and so give the body chance to rest and rebuild. As body
temperature starts to rise, it remains more difficult to stay in a deep sleep.
The skin
The top layer of the skin is made of closely packed
dead cells which are constantly shed during day. During deep sleep, the skin's
metabolic rate speeds up and many of the body's cells show increased production
and reduced breakdown of proteins.
Since proteins are the building blocks needed for
cell growth and for the repair of damage from factors like ultraviolet rays,
deep sleep may indeed be beauty sleep.
Daytime sleep will not compensate for loss of
nightly 'beauty sleep' as the energy needed for tissue repair is not available
during daylight because it is being used elsewhere.
Breathing
When we fall asleep, throat muscles relax so that
the throat gets narrower each time we inhale.
Snoring occurs when the throat is narrowed to a
slit and parts of the airway vibrate because of a resistance to breathing. Air
is forced through the narrow air passage, causing the soft palate and uvula to
vibrate.
Those who snore are more likely to have poor muscle
tone in the tongue and throat, which allows the tongue to fall backwards into
the airways. Being overweight or having large tonsils and adenoids also
contribute to snoring.
However, impaired breathing during sleep can cause
a condition called sleep apnoea.
Often caused by fat build-up, poor muscle tone or
ageing, sleep apnoea causes the windpipe to collapse when the muscles relax
during sleep. This blocks the air flow for from ten seconds to a minute while
the sleeping person struggles for breath.
When the person's blood oxygen level falls, the
brain responds by making the person tighten the upper airway muscles and open
the windpipe. This will result in a snort or gasp before snoring is resumed.
The mouth
Saliva is needed to lubricate the mouth and for
eating, but during sleep salivary flow is reduced, so causing a dry mouth in
the morning.
However, the mouth can be very active during sleep,
as one in 20 adults unconsciously grinds their teeth at night.
This is known as bruxism and occurs mainly during
stages one and two of sleep. It is caused by poor alignment of the teeth within
the jaw but is also thought to be nocturnal relief for daytime stresses.
Muscles
Though a person can change their sleeping position
about 35 times a night, the muscles of the body remain relaxed. This gives the
chance for tissues to be repaired and restored.
However, studies indicate that muscles might
receive just as much relaxation and repair during simple rest periods and that
a state of unconsciousness is not needed for this to take place.
Blood
The heart rate goes down by between 10 and 30 beats
per minute when we sleep. This produces a decline in blood pressure, which
occurs in restful sleep.
During rest, blood flows from the brain, distends
arteries and makes the limbs bigger.
Some scientists believe that fatigue-induced sleep
may be a mild form of blood detoxification.
This is because during the day debris from broken
down tissues is thrown into the bloodstream. In the waking state, most of this waste
will be eliminated through the lungs, kidneys, bowels and skin.
But there could be a saturation point. So nature
warns us to reduce these waste products to replenish lost energy, and that
causes us to get tired and sleep.
During sleep, the cells and tissues that break down
to produce toxic waste then become less active. This gives the chance for
broken-down tissue to be rebuilt.
Digestive system
The body requires a regular supply of energy and
its key source is glucose. This is constantly burned up to release energy for
muscle contraction, nerve impulses and regulating body temperature.
When we sleep, our need for these energy reserves
is marginal so the digestive system slows down to a sluggish pace. The
immobility of our bodies promotes this.
For this reason, eating late at night is not
recommended as our inactive state will prevent enzymes and stomach acids from
converting it to energy. This causes that telltale bloated feeling.
By Angel Epstein
