Psychology - Stages of sleep one to four
Stages of sleep one to four
This stage is characterised by small, irregular waves which are a
combination of alpha and theta waves.
During this stage the individual is drowsy and is drifting off into a
light sleep from which they can be easily awoken. The person's heart rate
slows down and their muscles relax.
Apart from the very first time the person enters stage one sleep, this
stage is known as REM sleep. 
In this stage sleep spindles  begin to occur. These are sharply
pointed waves recorded by the EEG. The EEG also shows rapid bursts of
electrical activity with irregular brain waves.
This stage is characterised by the onset of slower delta waves. These
brain waves are high in amplitude and low in frequency. At this stage, the
person becomes harder to awaken, their breathing and pulse rate slow down
and their temperature drops.
This stage consists of pure delta waves  and it is extremely
difficult to rouse the sleeper. This is the stage when sleep walking, 
sleep talking  and night terrors  occur.
Once the sleeper reaches stage four (about an hour after sleep begins)
they then travel back up through stages three, two and one. When stage one
is reached for the second time, REM sleep begins and the sleeper engages in
about ten minutes of dreaming.
Most dreams occur during this REM stage. The EEG reveals brain wave
patterns that are very similar to beta waves  when a person is awake,
active and alert. For this reason, REM sleep  has been called
'paradoxical sleep'. It is also characterised by jerky movements of the
eyes beneath the eyelids, hence the label, rapid eye movement. The person's
heart rate also increases, their blood pressure rises and their breathing
becomes faster and more irregular. Luckily for others, our muscles are
paralysed, preventing us from acting out our dreams.
As the night progresses, the time spent in stages three and four decreases
while REM sleep increases. The last stages or REM sleep  can last up to
one hour or so. The entire cycle of stages one through to four occurs
approximately four to six times during an average eight hour period of
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