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to be affected by many variables including the volume of
the meal, the meal constituents, and the circadian cycle of
the individual. In a follow-up study, 79 this same group
tested the hypothesis that afferent stimulation from the
gastric antrum would enhance postprandial sleepiness.
This was tested by comparing an equal volume distention
of the stomach with water to an equal caloric solid meal
and liquid meal. Sleep onset latency was determined sub-
sequent to each of these conditions. Because antral stimu-
lation results from the digestion of a solid meal, sleep onset
latency should be shorter subsequent to the consumption
of the solid meal. This was confirmed in this study in that
the sleep onset latency after the solid meal was significantly
shorter than the equal volume water condition. These
results are compatible with the animal studies cited earlier
and lend further support to the notion that contraction of
the lumen of the GI tract produces afferent stimulation,
which induces drowsiness.
First, it appears that spontaneous awakening does induce
high-amplitude peristaltic contractions as described by
Narducci and noted earlier, and this appears to be some-
what different than colonic motor activity induced by a
sudden awakening from sleep. In one study, sudden awak-
enings from sleep induced a pattern of segmental colonic
contractions, 88 rather than the propagating high-amplitude
peristaltic contractions noted in a previous study. 82 These
data are of considerable interest in that they demonstrate
not only the influence of higher cortical functions on
colonic motility but also the fact that the state of con-
sciousness can affect the colon in rather subtle ways in
terms of the induction of different patterns of colonic
The striated muscle of the anal canal was evaluated
during sleep in a 1953 study that included EEG docu-
mentation of sleep. 89 These investigators described a
marked reduction in EMG activity during sleep. They
concluded that this muscle is under voluntary control. In
another study, 90 anal canal pressure was measured continu-
ously during sleep, but without PSG monitoring. The
results indicated a decrease in the minute-to-minute varia-
tion and the amplitude of spontaneous decreases in anal
canal pressure during sleep. These results have been
largely confirmed in a study involving the ambulatory
monitoring of anorectal activity that noted a decrease in
anal canal resting pressure during sleep, as well as altera-
tions in rectal motor complexes (RMC) that were noted
to be similar in waking and sleep in the midrectum. 91
However, in the distal rectum, this motor pattern was
found to be more prevalent during sleep. They also noted
that during sleep, the anal canal resting pressure exceeded
that of the rectal pressure, which would seem to be impor-
tant in maintaining rectal continence during sleep. The
structures of the rectum and anal canal are vital in main-
taining normal bowel continence and ensuring normal
defecation. In general, normal defecation is associated with
sensory responses to rectal distention and appropriate
motor responses of the muscles of the anal canal. These
responses would include a contraction of the external anal
sphincter and a transient decrease in the internal anal
sphincter pressure associated with rectal distention. It is
thought that the high resting basal pressure in the internal
anal sphincter of the anal canal, as well as the response
of the external anal sphincter to rectal distention, is critical
in maintaining continence.
For assessment of the effect of sleep on these anorectal
sensorimotor responses, 10 normal volunteers were studied
during sleep with an anorectal probe in place. 92 This probe
permits the transient distention of the rectum via a rectal
balloon while the responses of the internal and external
anal sphincters can be simultaneously monitored. This
study documented a marked decrease—and, in most sub-
jects, an abolition—of the external anal sphincter response
to rectal distention. The internal anal sphincter response
remained unaltered. In addition, there was no evidence of
an arousal response with up to 50 mL of rectal distention
during sleep. The normal threshold of response in the
waking state is approximately 10 to 15 mL. These results
confirm that the external anal sphincter response to rectal
distention is most likely a learned response, whereas the
internal anal sphincter response is clearly a reflex response
The colon has two main functions: transport and absorp-
tion. These are critically determined by the motor activity
of the colon, which determines the rate of transport and,
therefore, indirectly, the rate of absorption from the
colonic lumen. Thus, alterations in colonic motility will
have significant consequences in terms of transit through
the colon and water absorption and ultimately clinical con-
sequences such as constipation and diarrhea.
In the early 1940s a study described a decrease in colonic
function during sleep. 80 These results have been confirmed
by two other studies that included measurements of the
transverse, descending, and sigmoid colon. 81 , 82 In one of
these studies, 82 a clear inhibition of colonic motility index
is evident during sleep in the transverse, descending, and
sigmoid colon segments, with a marked increase in activity
on awakening. Certainly, this explains the common urge
to defecate on awakening in the morning. Neither of these
studies attempted to document sleep with standard PSG.
However, in a subsequent study, 83 colonic activity from
cecum to rectum was monitored continuously for 32 hours,
and sleep was monitored via PSG. This study again noted
a rather marked decrease in colonic motor activity during
sleep, but it also described an interesting abolition in
propagating waves during slow-wave sleep. During REM
sleep, the frequency of propagating events rose substan-
tially. Other studies do not provide evidence for any sig-
nificant change in colonic motility or variability in the
rectosigmoid colon during sleep. 84 , 85 However, a study of
colonic myoelectrical activity in the human being sug-
gested a decrease in spike activity during sleep. 86 Again,
this study does not determine whether the results are
accounted for on the basis of true physiological sleep or
simply reflect a circadian variation in colonic activity inde-
pendent of sleep.
Collectively, these studies would suggest an inhibition
of colonic contractile and myoelectric activity during sleep,
and other studies have documented the fact that there is
diminished colonic tone during sleep. 87 Resumption of the
waking state, and consequently increased CNS arousal,
would suggest two different effects on colonic motility.
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