| Sleep: A Dynamic
Activity
Until the 1950s, most people thought of sleep as a passive,
dormant part of our daily lives. We now know that our
brains are very active during sleep. Moreover, sleep
affects our daily functioning and our physical and mental
health in many ways that we are just beginning to understand.
Nerve-signaling chemicals called neurotransmitters
control whether we are asleep or awake by acting on
different groups of nerve cells, or neurons, in the
brain. Neurons in the brainstem, which connects the
brain with the spinal cord, produce neurotransmitters
such as serotonin and norepinephrine that keep some
parts of the brain active while we are awake. Other
neurons at the base of the brain begin signaling when
we fall asleep. These neurons appear to "switch
off" the signals that keep us awake. Research also
suggests that a chemical called adenosine builds up
in our blood while we are awake and causes drowsiness.
This chemical gradually breaks down while we sleep.
During sleep, we usually pass through five phases
of sleep: stages 1, 2, 3, 4, and REM (rapid eye movement)
sleep. These stages progress in a cycle from stage 1
to REM sleep, then the cycle starts over again with
stage 1. We spend almost 50 percent of our total sleep
time in stage 2 sleep, about 20 percent in REM sleep,
and the remaining 30 percent in the other stages. Infants,
by contrast, spend about half of their sleep time in
REM sleep.
During stage 1, which is light sleep,
we drift in and out of sleep and can be awakened easily.
Our eyes move very slowly and muscle activity slows.
People awakened from stage 1 sleep often remember fragmented
visual images. Many also experience sudden muscle contractions
called hypnic myoclonia, often preceded by a sensation
of starting to fall. These sudden movements are similar
to the "jump" we make when startled.
When we enter stage 2 sleep, our eye
movements stop and our brain waves (fluctuations of
electrical activity that can be measured by electrodes)
become slower, with occasional bursts of rapid waves
called sleep spindles.
In stage 3, extremely slow brain
waves called delta waves begin to appear, interspersed
with smaller, faster waves.
By stage 4, the brain produces delta
waves almost exclusively. It is very difficult to wake
someone during stages 3 and 4, which together are called
deep sleep. There is no eye movement or muscle activity.
People awakened during deep sleep do not adjust immediately
and often feel groggy and disoriented for several minutes
after they wake up. Some children experience bedwetting,
night terrors, or sleepwalking during deep sleep.
When we switch into REM sleep, our
breathing becomes more rapid, irregular, and shallow,
our eyes jerk rapidly in various directions, and our
limb muscles become temporarily paralyzed. Our heart
rate increases, our blood pressure rises, and males
develop penile erections. When people awaken during
REM sleep, they often describe bizarre and illogical
tales-dreams.
The first REM sleep period usually
occurs about 70 to 90 minutes after we fall asleep.
A complete sleep cycle takes 90 to 110 minutes on average.
The first sleep cycles each night contain relatively
short REM periods and long periods of deep sleep. As
the night progresses, REM sleep periods increase in
length while deep sleep decreases. By morning, people
spend nearly all their sleep time in stages 1, 2, and
REM.
People awakened after sleeping more than a few minutes
are usually unable to recall the last few minutes before
they fell asleep. This sleep-related form of amnesia
is the reason people often forget telephone calls or
conversations they've had in the middle of the night.
It also explains why we often do not remember our alarms
ringing in the morning if we go right back to sleep
after turning them off.
Since sleep and wakefulness are influenced by different
neurotransmitter signals in the brain, foods and medicines
that change the balance of these signals affect whether
we feel alert or drowsy and how well we sleep. Caffeinated
drinks such as coffee and drugs such as diet
pills and decongestants stimulate some parts of
the brain and can cause insomnia, or an inability to
sleep. Many antidepressants suppress REM sleep.
Heavy smokers often sleep very lightly and have reduced
amounts of REM sleep. They also tend to wake up after
3 or 4 hours of sleep due to nicotine withdrawal. Many
people who suffer from insomnia try to solve the problem
with alcohol-the so-called nightcap. While alcohol does
help people fall into light sleep, it also robs them
of REM and the deeper, more restorative stages of sleep.
Instead, it keeps them in the lighter stages of sleep,
from which they can be awakened easily.
People lose some of the ability to regulate their
body temperature during REM, so abnormally hot or cold
temperatures in the environment can disrupt this stage
of sleep. If our REM sleep is disrupted one night, our
bodies don't follow the normal sleep cycle progression
the next time we doze off. Instead, we often slip directly
into REM sleep and go through extended periods of REM
until we "catch up" on this stage of sleep.
People who are under anesthesia or in a coma are often
said to be asleep. However, people in these conditions
cannot be awakened and do not produce the complex, active
brain wave patterns seen in normal sleep. Instead, their
brain waves are very slow and weak, sometimes all but
undetectable.
About the Author
Chris Read, An associated editor to Hateweight.com.
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