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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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