SLEEP AND WAKEFULNESS
Overview (P. 497)
- Sleep is defined behaviorally by the suspension
of normal consciousness and electrophysiologically by specific
brain wave criteria.
- Sleep consumes fully a third of our lives - what
really happens to our body while we sleep?
- One thing for sure is, sleep is not the result
of simple diminution of brain activity; rather, sleep is a series
of precisely controlled brain states.
- To understand this concept, let us follow the
brain activity, by electroencephalogram (EEG; Box A) in a normal
human (Fig. 26.1).
- The waking state is characterized by high-frequency,
low amplitude activity. These waves are called beta
(eyes open) and alpha (eyes closed) waves.
- This state is followed by non-rapid eye movement
(non-REM) sleep; which in itself has 4 stages.
- Non-REM sleep is characterized by decreasing
frequency and increasing amplitude of EEG waves.
These waves are called slow-waves or delta waves.
- Following non-REM sleep, the person's EEG goes
back to low-voltage, high-frequency activity
that is remarkably similar to the EEG activity of individuals
who are awake. This stage is called REM sleep.
- A typical night's sleep is shown in Fig. 26.2.
In total then, the typical 8 hours of sleep is divided into about
1.5 to 2 hours of REM sleep and about 6 hours
of non-REM sleep.
Non-REM Sleep (P.
- There are several evidence that suggest that
non-REM sleep is restorative in function.
- First, the metabolism of the brain during slow-wave
sleep, as measured by cerebral blood flow, is reduced by as much
as 45% (Fig. 26.3).
- Second, non-REM sleep deprivation in lab animals
eventually lead to a breakdown of homeostatic function, and ultimately
- Despite the fact that the brain is relatively
quiescent during the non-REM sleep, the body is remarkably active.
This is the period when sleepwalking commonly occurs.
REM Sleep (P. 499)
- During REM sleep the eyes move rapidly; hence
the name rapid eye movement sleep.
- The body does not respond to stimuli during this
stage, despite the fact that the brain is active.
- The REM sleep waves originate in the pontine
reticular formation and propagate through the lateral geniculate
nucleus of the thalamus, and then to the occipital cortex.
- These pontine-geniculo-occipital (PGO)
waves therefore provide a useful marker for the beginning of REM
sleep, although the specific purpose of the PGO waves is not clear.
- Interestingly, the overall duration of REM sleep
varies as a function of age (Fig. 26.4).
- High proportion of fetal and infant sleep is
REM sleep - which has provoked a good deal of speculation, that
learning is linked to REM sleep.
- Most of the dreams occur during REM sleep. What
is the function of dreams any way?
- An imaginative hypothesis by Francis Crick is:
The function of dreams is to act as an "unlearning"
mechanism, whereby certain modes of neural activity are erased
by random activation.
- Roughly speaking, unwanted thoughts or erroneous
information, which, if not expunged, might become the basis for
obsession, paranoia, or other thought pathology
- Although REM sleep occurs in nearly all mammals.
Birds, reptiles and amphibians lack it.
- Even few mammals like the spiny anteater,
which has a remarkably large cerebral cortex, does not show any
signs of REM sleep. (the big brain may be to compensate the inability
to expunge parasitic mode of thoughts).
- Deprivation of REM sleep in humans for as much
as 2 weeks has little or no effect on behavior.
- Similarly, patients taking certain antidepressants
have little or nor REM sleep, yet show no obvious ill effects,
even after months or years of treatment.
- Human sleep occurs with circadian periodicity.
- What happens, for example, if one is prevented
from sensing cues they normally have about night and day?
- The normal circadian rhythm is maintained, but
slightly loses its usual relationship to the actual time. For
example, a day becomes 25 hours, instead of the normal 24 (Fig.
- Thus, humans (and many mammals) have an internal
clock that continues to operate in the absence of any external
information about the time of day.
- In animals, this clock is the suprachiasmatic
nucleus (SCN) of the hypothalamus (Fig. 26.7 B).
- Removal of SCN in animals abolishes their circadian
- Recent findings suggest that cells in the SCN
contain oscillatory molecular mechanisms that control expression
of specific gene products, effectively creating a timing mechanism
for circadian rhythm.
- Not surprisingly, SCN receives info from the
retina, and is connected to the brain structures important in
controlling sleep and wakefulness (see below).
Brainstem Mechanisms of Sleep and Wakefulness
- Sleep and wakefulness are now understood to be
generated by the activation of specific neural centers.
- Electrical stimulation of the midbrain reticular
formation, called reticular activating system (RAS)
causes a state of wakefulness and arousal.
- RAS is a group of nuclei near the pons-midbrain
junction (Fig. 26.8).
- Many of the neurons in these nuclei have high
discharge rates during waking and in REM sleep; conversely they
are quiescent during non-REM sleep.
- These nuclei have widespread ascending and descending
connections to other regions of the brain, which explains their
- This observation implied that wakefulness requires
a special mechanism, not just presence of adequate sensory experience.
- A quite different idea about the mechanism of
sleep is that one or more molecules builds up in the brain during
wakefulness and induce sleep. Although, not widely accepted, this
idea has led to the identity of bacterial muramyl dipeptide as
the sleep factor in humans (Box C).