214 Chapter 8
Delta waves are seemingly emitted in a general pattern
from the cerebral cortex. These waves have a frequency of 1 to
5 cycles/second and are common during sleep and in an awake
infant. The presence of delta waves in an awake adult indicates
brain damage.Sleep
Although environmental factors affect sleep, there is evidence
that sleep is genetically controlled. This is shown by sleep dis-
orders that run in families and the heritability of sleep patterns.
Histamine and several other neurotransmitters promote wake-
fulness, while adenosine and GABA promote sleep. The neural
control of sleep and arousal is discussed in conjunction with
the reticular activating system in section 8.4.
Two categories of sleep are recognized. Dreams—at least
those that are vivid enough to recall upon waking—occur dur-
ing rapid eye movement (REM) sleep. The name describes
the characteristic eye movements that occur during this stage of
sleep. The remainder of the time sleeping is spent in non-REM,
or resting, sleep. These two stages of sleep can also be distin-
guished by their EEG patterns. The EEG pattern during REM
sleep consists of theta waves (5 to 8 cycles per second), although
the EEG is often desynchronized as in wakefulness. Non-REM
sleep is divided into four stages based on the EEG patterns;
stages 3 and 4 are also known as slow-wave sleep, because of
their characteristic delta waves (1 to 5 cycles per second).
When people first fall asleep, they enter non-REM sleep of
four different stages, and then ascend back through these stages
to REM sleep. After REM sleep, they again descend through the
stages of non-REM sleep and back up to REM sleep. Each of
these cycles lasts approximately 90 minutes, and a person may
typically go through about five REM-to-non-REM cycles a night.
A great amount of time is spent in slow-wave sleep during the
first half of a night’s sleep; this gives way to mostly REM sleep
during the second half of the sleep. When people are allowed to
wake up naturally, they generally awaken from REM sleep.
Most neurons decrease their firing rate in the transi-
tion from waking to non-REM sleep. This correlates with a
decreased energy metabolism and blood flow, as revealed by
PET studies. By contrast, REM sleep is accompanied by a
higher total brain metabolism and by a higher blood flow to
selected brain regions than in the waking state. Interestingly,
the limbic system (described shortly) is activated during REM
sleep. The limbic system is involved in emotions, and part of it,
the amygdala, helps to mediate fear and anxiety. Because these
are common emotions during dreaming, it makes sense that the
limbic system would be active during REM sleep.
During non-REM sleep, the breathing and heart rate tend
to be very regular. In REM sleep, by contrast, the breathing
and heart rate are as irregular as they are during waking. This
may relate to dreaming and the activation of the brain regions
involved in emotions during REM sleep.
Non-REM sleep aids the neural plasticity required for
learning. For example, subjects allowed to have non-REM
sleep after a learning trial displayed improved performanceplaced on the scalp. Deviations from normal EEG patterns can
be used clinically to diagnose epilepsy and other abnormal
states, and the absence of an EEG can be used to signify brain
death.
There are normally four types of EEG patterns ( fig. 8.10 ).
Alpha waves are best recorded from the parietal and occipi-
tal regions while a person is awake and relaxed but with the
eyes closed. These waves are rhythmic oscillations of 10 to 12
cycles/second. The alpha rhythm of a child under the age of 8
occurs at a slightly lower frequency of 4 to 7 cycles/second.
Beta waves are strongest from the frontal lobes, especially
the area near the precentral gyrus. These waves are produced
by visual stimuli and mental activity. Because they respond to
stimuli from receptors and are superimposed on the continuous
activity patterns, they constitute evoked activity. Beta waves
occur at a frequency of 13 to 25 cycles per second.
Theta waves are emitted from the temporal and occipital
lobes. They have a frequency of 5 to 8 cycles/second and are
common in newborn infants and sleeping adults. Theta rhythms
increase in awake adults during tasks that require attention and
memory, during sleep deprivation, and during severe emotional
stress.
Figure 8.10 Different types of waves in an electro-
encephalogram (EEG). Notice that the delta waves ( bottom )
have the highest amplitude and lowest frequency.
AlphaBetaThetaDelta1 sec