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.
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 eye 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 night cap. 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 anaesthesia 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.
Source:
Yahoo Health