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- Mankind has always been intrigued by sleep
and dreams, and has researched their importance.
Interpretation of dreams and their importance
has created a whole new "profession". Sleepiness
and yawning precede sleep, therefore yawning,
sleeping and dreams are actions directly related
to one another. Yawning is an action seen at
every stage of life, from early embryonic to old
age. (Fig. 1). Yawning is also seen among
animals: in mammals, birds and reptiles (Fig.
2). This behaviour leads to the conclusion of a
common forefather, from which the different
species have evolved, keeping certain
behaviours, like yawning, despite their evolving
in different directions.
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- Ontogenetically yawning is involuntary, and
a stereotyped behaviour, therefore it is
considered a physiologic reflex. The reflex
uses: the hypothalamus, the brain stem reticular
net (Formatio reticularis), portion of
respiratory neurons in the medulla, parts of
motor nerves V, VII. IX. and X, the Nervus
phrenicus and the intercostal nerves.
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- Yawning and mouth/facial movements are
controlled by the V. and VII. Cranial nerves. In
utero yawning differs from mature yawning, since
the tongue is extended. The opening of the mouth
can be seen from the 10th week of pregnancy,
whereas the mouth-reflex is seen in the 18th
week. Until the 15th week the wide opening of
the jaw is more regular than later. The
irregular repeated movements are however seen
more often after the 15th week, than before. The
opening of the jaw can happen solo, or be
accompanied by hickups or isolated movements of
the head.
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- The embryo tilts its head backwards, and
only then opens its mouth. The neck and the
mouth are slightly extended, and then retracted.
The tongue is meanwhile pushed down and to the
back of the mouth. The extended wide opening of
the mouth takes two to eight seconds followed by
a quick closing of the mouth (4, 5, 6, 7). The
opening of the jaw can be slow or fast.
Sometimes concommitant lifting of the arms can
be witnessed, while the head ante flexes, the
tongue retracts and the mouth shuts quickly.
This takes place within one to five seconds, and
can happen once, or repeatedly. Normal embryos
will yawn 5 times an hour, sick, retarded or
anemic will yawn more often.
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- Yawning also has some suggestive effect. If
one yawns, others present at the scene will yawn
too, regardless of whether he is tired or not.
It is interesting to note that yawning can occur
during sleep (Fig.3). Sleep plays an important
role in our health and in our life. Significant
parts of our life are occupied by periods of
sleep. The younger the subject, the longer the
sum of the periods of sleep will be. Embryos
will spend about 40% of their time in the womb
sleeping (Fig. 4). Once born this decreases, but
still a significant amount of time is spent
sleeping. The elderly sleep the least. The
ancient Romans were already aware of the
importance of sleep in their lives. They left us
the saying: "sex horas satis est" (six hours are
sufficient), but they did not count the
after-lunch nap.
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