vigorous eye movements and virtually complete loss of muscle tone, except in the
muscles of respiration. The first bout of REM sleep during the night typically lasts
only 5 to 10 minutes, and then the subject will transition into stage I NREM, and
again begin to descend gradually into deeper stages of NREM sleep.
As the night progresses, the subject typically will spend progressively less time in
the deeper stages of NREM sleep, and more time in REM sleep, so that most of the
REM sleep for the night comes in the last few bouts. Spontaneous awakenings during
the night typically occur from the lighter stages of NREM sleep. Active dreams
Hours of night
1
2
3
4
5
6
7
4
3
2
1
REM
Awake
Old age
1
2
3
4
5
6
7
4
3
2
1
REM
Awake
Sleep stages
Early adulthood
1
2
3
4
5
6
7
4
3
2
1
REM
Awake
Childhood
Figure B1–3B. The stages of sleep through the night in a child, young adult, and older person. There is
usually regular progression from wakefulness through the stages of non-rapid eye movement (NREM)
sleep into its deepest stages, then progression back to light NREM sleep before the first REM episode
of the night. With successive cycles through the night, the amount of deeper NREM sleep becomes
less, and the amount of REM becomes greater. With aging, the amount of deep NREM sleep dimi-
nishes, and sleep fragmentation with more frequent awakenings is seen. (From Rechtschaffen, A, and
Siegel, J. Sleep and dreaming. Chapter 47 in: Kandel, ER, Schwartz, JH, Jessel, TM. Principles of Neural
Science. 4th ed. McGraw-Hill, New York, 2000, pp. 936–947. By permission of McGraw-Hill.)
(continued)
17
by Lu and Saper (unpublished) have focused on
neurons in the mesopontine tegmentum that
provide inputs to the basal forebrain, which is
critical for maintaining a wakeful state. Popula-
tions of neurons in the pre-locus coeruleus area
and medial parabrachial nucleus have intense in-
puts to the basal forebrain. Cell-specific lesions
of these neurons produce profound coma, sug-
gesting that they may be a major source of the
ascending arousal influence.
In addition, along the course of the ascending
cholinergic and monoaminergic axons through
the rostral midbrain reticular formation, there
are many additional neurons that project to the
thalamic relay, midline, and intralaminar nu-
clei.
34
Most of these neurons appear to be gluta-
matergic, and they may amplify the arousal signal
that arises in the mesopontine tegmentum. On
the other hand, they do not appear to be cap-
able of maintaining a waking state in the case of
acute loss of the influence from the mesopontine
neurons.
Along the course of the ascending arousal
systems, as they pass through the hypothala-
mus, are several hypothalamic cell groups that
augment the ascending projection to the basal
forebrain and cerebral cortex. These include his-
taminergic neurons in the tuberomammillary
nucleus as well as several populations of neu-
rons in the lateral hypothalamic area, all of which
project diffusely to the cerebral cortex and
innervate the intralaminar and midline thala-
mus.
54
There is considerable evidence that the
histaminergic input in particular is important
for maintaining a wakeful state. Histamine H
1
blockers impair wakefulness in both animals
and humans,
55
and transgenic mice lacking H
1
receptors have impairment of arousal responses
induced by intraventricular injection of the pep-
tide orexin.
56
Transgenic mice lacking histidine
decarboxylase show a deficit in wakefulness in-
duced by a novel environment, and mice in-
jected with an inhibitor of this key enzyme for
histamine synthesis similarly show less wake-
fulness.
57
Some of the lateral hypothalamic neurons
contain orexin,
58
a peptide that is associated
with arousal, and others contain melanin-
concentrating hormone
59,60
or GABA.
61
Many
neurons in the lateral hypothalamic area, in-
cluding those that contain orexin, fire fastest
during wakefulness and slow down during
both slow-wave and REM sleep.
62,63
Alterna-
tively, the firing of some lateral hypothalamic
neurons, which are likely to contain melanin-
concentrating hormone, increases during REM
sleep.
38,64,65
In addition, the ascending monoaminergic
and hypothalamic projections pass through the
basal forebrain, and along their pathway to the
cerebral cortex, they encounter and are aug-
mented further by additional populations of
occur predominantly during REM sleep, although many subjects report passive
dreams and ideation during NREM sleep as well.
This pattern, which is typical of young adults, changes dramatically across a life-
time. Infants spend much more time asleep, and much more time in the deeper
stages of NREM sleep, than adults. The amount of stages III and IV NREM sleep
diminishes as children enter puberty, and it may not occur at all in some older
adults. Thus, phenomena such as night terrors, bed wetting, and sleep walking tend
to occur mainly during slow-wave sleep in children but disappear as the children
become older and spend less time in those sleep stages. Most sedative drugs are
GABA
A
receptor agonists that acutely increase the amount of time spent in the
lighter stages of NREM sleep, but there may be little time spent in stages III or IV
of NREM or in REM sleep. These drugs are thought to act directly on the arousal
system, inhibiting the firing of its neurons. Newer drugs such as gaboxadol, which
acts on a specific class of GABA
A
receptors containing delta subunits, may allow
activation of the endogenous sleep system of the brain, and produce a pattern of
sleep including more deep slow waves and more REM sleep.
Box 1–3 Wake-Sleep States (cont.)
18
Plum and Posner’s Diagnosis of Stupor and Coma