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Sleep is a behavioural state characterized by little
physical activity and almost no awareness of the outside world. Most
scientists think that sleep does something important -- something
vital for life, although research has not yet identified specifically
what sleep does. Nevertheless, we all know when we need to sleep --
we can feel this need. We also know when sleep has done its work --
we feel rested and that we have slept enough. Another important feature
of normal sleep is that it can end quickly. Although a sleeper may
appear to be unconscious; unlike someone who is actually knocked-out,
anesthetized or in a coma; a sleeping person can be easily awakened
and can resume normal waking activity within a minute or two.
Sleep is an active, highly organized sequence of events and physiological
conditions. Sleep is actually made up of two separate and distinctly
different states: 'non-rapid eye movement sleep (NREM sleep) and
'rapid eye movement’ sleep (REM sleep) or dreaming sleep.
The NREM and REM types of sleep are as different from one another
as both are different from wakefulness.
NREM sleep is further divided into stages 1 - 4 based on the size
and speed of the brain waves generated by the sleeper. Stages 3
and 4 of NREM sleep have the biggest and slowest brain waves. These
big, slow waves are called delta waves and stages 3 and 4 sleep
combined are often called 'slow-wave sleep' or 'delta sleep'.
During REM sleep you can watch the sleeper's eyes move around beneath
closed eyelids. Some scientists think that the eyes move in a pattern
that relates to the visual images of the dream. We are almost completely
paralyzed in REM sleep -- only the heart, diaphragm, eye muscles
and the smooth muscles (such as the muscles of the intestines and
blood vessels) are spared from the paralysis of REM sleep.
Doctors have tried to determine what type of sleep is the deepest
sleep. To do this, they measure how much noise or other alerting
stimulation is required to awaken a sleeper from the various types
of sleep. It is always possible to awaken someone who is sleeping,
as opposed to, say, someone who is in a coma. However, people in
stages 3 and 4 sleep require the most stimulation to awaken. Therefore,
this phase of sleep is often thought of as 'deep sleep'. Also, large
spurts of growth hormone are secreted during stages 3 and 4 NREM
sleep. Consequently, these stages of sleep are thought to restore
the body from the wear and tear of waking activity.
People in REM sleep also tend to be quite difficult to awaken,
but this finding is variable -- sometimes even the slightest noise
can awaken a person in REM sleep. Nevertheless, because it is often
difficult to awaken a person from REM sleep, many doctors think
also of REM sleep as a 'deep' phase of sleep.
There are many theories about the function of REM sleep and dreaming
-- ranging from 'safe, socially acceptable, wish fulfilment' to
'consolidation of memories' to 'providing necessary stimulation
to the entire nervous system during development'. Researchers used
to think that REM sleep was necessary for normal psychological function,
because experimental REM deprivation caused some subjects to behave
strangely. The notion that we need REM sleep for our mental health
is not accepted now, because, among other reasons, people have uneventfully
withstood long and almost complete REM deprivation. Some experiments
have shown that REM deprivation improves depression. However, REM
sleep must still do something, because rats will die after 2 - 3
weeks if they are deprived of REM sleep by a special experimental
computer that wakes them up each time REM sleep is achieved. Whatever
REM sleep does, it is clear that every aspect of existence, from
the body's manufacture of proteins to sexual arousal including orgasm,
is influenced by REM sleep. It is likely that the ultimate explanation
of REM sleep will be very broad -- not simply focused on one physiologic
function
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