Consciousness, the Brain, and Behavior
235
cycles, each lasting 90 to 100 min (
Figure 8–5
). In young
adults, REM sleep constitutes 20 to 25 percent of the total
sleeping time; this fraction tends to decline progressively with
aging. The time spent in REM sleep increases toward the end
of an undisturbed night. Initially, as one moves from drows-
iness to stage 1 sleep, there is a considerable tension in the
postural muscles, but the muscles become progressively more
relaxed as NREM sleep progresses. Sleepers awakened during
NREM sleep rarely report dreaming.
With several exceptions, skeletal muscle tension, already
decreased during NREM sleep, is markedly inhibited during
REM sleep. Exceptions occur in the eye muscles, which cause
the rapid bursts of sweeping eye movements (see Figure 8–5),
and the motor neurons to the muscles of respiration. In one
form of the disease known as
sleep apnea,
however, stimulation
of the respiratory muscles temporarily ceases, sometimes hun-
dreds of times during a night. The resulting decreases in oxygen
levels repeatedly awaken the apnea sufferer, who is deprived of
slow-wave and REM sleep. As a result, this disease is associated
with excessive—and sometimes dangerous—sleepiness during
the day (refer to Chapter 13 for a more complete discussion of
sleep apnea).
During the sleep cycle, many changes occur through-
out the body, in addition to altered muscle tension. During
NREM sleep, for example, there are pulsatile releases of growth
hormone and the gonadotropic hormones from the anterior
pituitary (Chapter 11), as well as decreases in blood pressure,
heart rate, and respiratory rate. REM sleep is associated with
an increase and irregularity in blood pressure, heart rate, and
respiratory rate. Moreover, twitches of the facial muscles or
limb muscles may occur (despite the generalized lack of skel-
etal muscle tone), as may erection of the penis and engorge-
ment of the clitoris.
Although we spend about one-third of our lives sleep-
ing, the functions of sleep are not completely understood.
Many lines of research, however, suggest that it is a fundamen-
tal necessity of a complex nervous system. Sleep, or a sleep-
like state, is a characteristic found throughout the animal
kingdom, from organisms as simple as fruit fl ies to the most
complex primates. Studies of sleep deprivation in humans and
other animals suggest that it is a homeostatic requirement,
similar to the need for food and water. Lack of sleep impairs
the immune system, causes cognitive and memory defi cits, and
ultimately leads to psychosis and even death. Much of the sleep
research on humans has focused on the importance of sleep
for learning and memory formation. EEG studies show that
during sleep the brain experiences reactivation of neural path-
ways stimulated during the prior awake state, and that sub-
jects deprived of sleep show less effective memory retention.
Based on these and other fi ndings, many scientists believe that
part of the restorative value of sleep lies in facilitating chemical
and structural changes responsible for dampening the overall
activity in the brain’s neural networks while conserving and
strengthening synapses in pathways associated with informa-
tion that is important to learn and remember.
Table 8–1
summarizes the sleep states.
Neural Substrates of States of Consciousness
Periods of sleep and wakefulness alternate about once a day;
that is, they manifest a circadian rhythm consisting on average
of 8 h sleep and 16 h awake. Within the sleep portion of this
circadian cycle, NREM sleep and REM sleep alternate, as we
have seen. As we shift from the waking state through NREM
sleep to REM sleep, attention shifts to internally generated
stimuli (dreams) so that we are largely insensitive to external
stimuli. Memory decreases (dreams are generally forgotten
much faster than events we experience while awake), and pos-
tural muscles lose tone so our heads nod and we slump in our
chairs. The tight rules for determining what is real become
relaxed, allowing the bizarre happenings of our dreams.
What physiological processes drive these cyclic changes
in states of consciousness? Nuclei in both the brainstem and
hypothalamus are involved.
Recall from Chapter 6 that neurons of certain brainstem
nuclei give rise to axons that diverge to affect wide areas of
1
02
3
4
5
68
7
Number
of eye
movements
Number of
neck muscle
movements
Heart rate
Respiration
rate
Time (hours)
Stage 1
Stage 2
Stage 3
Stage 4
Sleep
Awake
Figure 8–5
Schematic representations of the EEG and other physiological
changes associated with the stages of sleep. Purple bars indicate
periods of REM sleep. During periods of REM sleep, muscle
movements decrease, while ocular movements, heart rate, and
respiration rate increase.
previous page 263 Vander's Human Physiology The Mechanisms of Body Function read online next page 265 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off