Sleep
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.
Next: How much sleep do we need? »
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