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 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 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.