Regulation and Integration of the Body
Te alternating cycles of sleep and wakefulness reﬂect a natural
, or 24-hour,
. Te hypothalamus is responsible
for the timing of the sleep cycle. Its
biological clock) regulates its
center). By inhibiting the brain stem’s reticular activating system
(RAS; see Figure 12.17), the preoptic nucleus puts the cerebral
cortex to sleep. However, sleep is much more than simply turn-
ing oﬀ the arousal system. RAS centers not only help maintain
the awake state but also mediate some sleep stages, especially
or infections that invade the brain stem, (3) metabolic distur-
bances such as hypoglycemia (abnormally low blood sugar
levels), (4) drug overdose, (5) liver or kidney failure. Strokes
rarely cause coma unless they are massive and accompanied by
extreme swelling of the brain, or are located in the brain stem.
When the brain has suﬀered irreparable damage, irreversible
coma occurs, even though life-support measures may restore
vitality to other body organs. Te result is
, a dead
brain in an otherwise living body. Because life support can be
removed only a±er death, physicians must determine whether a
patient in an irreversible coma is legally alive or dead.
Sleep and Sleep-Wake Cycles
Compare and contrast the events and importance of slow-
wave and REM sleep, and indicate how their patterns
change through life.
Sleep is deﬁned as a state of partial unconsciousness from which
a person can be aroused by stimulation. Tis distinguishes sleep
from coma, a state of unconsciousness from which a person
be aroused by even the most vigorous stimuli.
For the most part, cortical activity is depressed during sleep,
but brain stem functions continue, such as control of respira-
tion, heart rate, and blood pressure. Even environmental moni-
toring continues to some extent, as illustrated by the fact that
strong stimuli (“things that go bump in the night”) immediately
arouse us. In fact, people who sleepwalk can avoid objects and
navigate stairs while truly asleep.
Types of Sleep
Te two major types of sleep, which alternate through most of
the sleep cycle, are
non–rapid eye movement (NREM) sleep
rapid eye movement (REM) sleep
, deﬁned in terms of their EEG
. During the ﬁrst 30 to 45 minutes of the
sleep cycle, we pass through the ﬁrst two stages of NREM sleep
and into NREM stages 3 and 4, also called
. As we
pass through these stages and slip into deeper and deeper sleep,
the frequency of the EEG waves declines, but their amplitude in-
creases. Blood pressure and heart rate also decrease.
About 90 minutes a±er sleep begins, a±er reaching NREM stage
4, the EEG pattern changes abruptly. It becomes very irregular and
appears to backtrack quickly through the stages until alpha waves
(more typical of the awake state) reappear, indicating the onset of
REM sleep. Tis brain wave change is coupled with increases in
heart rate, respiratory rate, and blood pressure and a decrease in
gastrointestinal motility. Oxygen use by the brain is tremendous
during REM—greater than during the awake state.
Although the eyes move rapidly under the lids during REM,
most of the body’s skeletal muscles are actively inhibited and go
limp. Tis temporary paralysis prevents us from acting out our
dreams. Most dreaming occurs during REM sleep, and some
suggest that the ﬂitting eye movements are following the visual
imagery of our dreams. (Note, however, that most nightmares
and night terrors occur during NREM stages 3 and 4.) In ad-
olescents and adults, REM episodes are frequently associated
with erection of the penis or engorgement of the clitoris.
(b) Typical progression of an adult through one night’s sleep stages
(a) Typical EEG patterns
Skeletal muscles (except
ocular muscles and diaphragm)
are actively inhibited; most
NREM stage 1:
begins; EEG shows alpha waves;
arousal is easy.
NREM stage 2:
with sleep spindles (short high-
amplitude bursts); arousal is more
NREM stage 3:
theta and delta waves appear;
vital signs decline.
NREM stage 4:
dominated by delta waves;
arousal is difficult; bed-wetting,
night terrors, and sleepwalking
Types and stages of sleep.
The four stages of
non–rapid eye movement (NREM) sleep and rapid eye movement
(REM) sleep are shown.