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The Sleep
Cycle
One
sleep cycle comprises of four stages and lasts for about 90-120
minutes. Note that some books list five stages in the sleep cycle.
These books consider the first five to ten minutes when you are falling asleep as a
stage in the sleep cycle. We think this is more of a transitional phase and
not really part of the cycle, especially since this stage of sleep does
not repeat itself, while the other four stages do repeat themselves throughout the
night. For this
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reason,
we have excluded as part of the cycle.
Dreams can occur
in any of the four stages of sleep, but the
most vivid and memorable dreams occur in the last stage of sleep
(also commonly referred to as REM sleep). The sleep cycle repeats itself
about an average of four to five times per night, but may repeat as many
as seven times. Thus, you can see how a person has several different
dreams in one night. However, most people only
remember dreams that
occur closer toward the morning
when they are about to wake up. But just because you can't remember
those dreams do not mean that they never happened. Some people
believe that they simply do not
dream, when in reality, they just don't remember their dreams.
The
Stages Of Sleep
The
stages in the sleep cycle are organized by the changes in specific brain
activity.
Stage
1: You are entering
into light sleep. This stage is characterized by Non-rapid eye movements (NREM),
muscle relaxation, lowered body temperature and slowed heart rate. The body is preparing to
enter into deep sleep.
Stage
2: Also
characterized by NREM, this stage is characterized by a further drop in body
temperature and relaxation of the muscles. The body's immune system goes
to work on repairing the day's damage. The endocrine glands secrete growth
hormones, while blood is sent to the muscles to be reconditioned. In this
stage, you are completely asleep.
Stage
3: Still in the
NREM stage, this is an even deeper sleep. Your metabolic levels are
extremely slow.
Stage
4: In this stage of
sleep, your eyes move back and forth erratically as if watching something
from underneath your eyelids. Referred to as REM sleep
or delta sleep,
this stage occurs at about 90-100 minutes after the onset of sleep.
Your blood pressure rises, heart rate speeds up, respiration becomes
erratic and brain activity increases. Your involuntary muscles also become
paralyzed or immobilized. This stage is the most restorative part of sleep.
Your mind is being revitalized and emotions is being fine tuned. The majority of your dreaming
occurs in this stage. If you are awakened during this stage of sleep, you
are more likely to remember your dreams.
These
stages repeat themselves throughout the night as you sleep. As the cycle
repeats, you will spend less time in stages 1 to 3 and more time
dreaming in stage 4. In other words, it will be quicker and quicker
for you to get to stage 4 each time the cycle repeats.
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