How To Health

Sleep was long considered just a uniform block of time when you
are not awake. Thanks to sleep studies done over the past several
decades, it is now known that sleep has distinct stages that cycle
throughout the night in predictable patterns. How well rested you are
and how well you function depend not just on your total sleep time
but on how much of the various stages of sleep you get each night.
Your brain stays active throughout sleep, and each stage of sleep
is linked to a distinctive pattern of electrical activity known as
brain waves.

Sleep is divided into two basic types: rapid eye movement (REM)
sleep and non-REM sleep (with four different stages). (See “Types
of Sleep” on page 5.) Typically, sleep begins with non-REM sleep.
In stage 1 non-REM sleep, you sleep lightly and can be awakened
easily by noises or other disturbances. During this first stage of
sleep, your eyes move slowly, and your muscle activity slows. You
then enter stage 2 non-REM sleep, when your eye movements stop.
Your brain shows a distinctive pattern of slower brain waves with
occasional bursts of rapid waves.

When you progress into stage 3 non-REM sleep, your brain waves
become even slower, although they are still punctuated by smaller,
faster waves. By stage 4 non-REM sleep, the brain produces
extremely slow waves almost exclusively. Stages 3 and 4 are considered
deep sleep, during which it is very difficult to be awakened.
Children who wet the bed or sleep walk tend to do so during stages
3 or 4 of non-REM sleep. Deep sleep is considered the “restorative”
part of sleep that is necessary for feeling well rested and energetic
during the day.

During REM sleep, your eyes move rapidly in various directions,
even though your eyelids remain closed. Your breathing also
becomes more rapid, irregular, and shallow, and your heart rate and

blood pressure increase. Dreaming typically occurs during REM
sleep. During this type of sleep, your arm and leg muscles are temporarily
paralyzed so that you cannot “act out” any dreams that
you may be having.

The first period of REM sleep you experience usually occurs about
an hour to an hour and a half after falling asleep. After that, the
sleep stages repeat themselves continuously while you sleep. As the
night progresses, REM sleep time becomes longer, while time spent
in non-REM sleep stages 3 and 4 becomes shorter. By morning,
nearly all your sleep time is spent in stages 1 and 2 of non-REM
sleep and in REM sleep. If REM sleep is disrupted during one night,
REM sleep time is typically longer than normal in subsequent nights
until you catch up. Overall, almost one-half your total sleep time is
spent in stages 1 and 2 non-REM sleep and about one-fifth each in
deep sleep (stages 3 and 4 of non-REM sleep) and REM sleep. In
contrast, infants spend half or more of their total sleep time in REM
sleep. Gradually, as they mature, the percentage of total sleep time
they spend in REM progressively decreases to reach the one-fifth
level typical of later childhood and adulthood.

Why people dream and why REM sleep is so important are not well
understood. It is known that REM sleep stimulates the brain regions
used in learning and the laying down of memories. Animal studies
suggest that dreams may reflect the brain’s sorting and selectively
storing important new information acquired during wake time. While
this information is processed, the brain might revisit scenes from the
day while pulling up older memories. This process may explain why
childhood memories can be interspersed with more recent events
during dreams. Studies show, however, that other stages of sleep
besides REM are also needed to form the pathways in the brain that
enable us to learn and remember.

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