Yawning: A Brief Survey of the
Main Possible Explanations
Yawning is a universal phenomenon shared
by most vertebrate species from fetal life to
old age.
Yawning is a common symptom of several
illnesses, ranging from various pathologies of
the central nervous system to diabetes and
diseases of the liver and the adrenals, and as
such has been studied predominantly in relation
to these. Spontaneous yawns, however, have
intrigued many over the centuries and numerous
theories have been advanced as to why we yawn
ordinarily. Up to this day, however, the
function of spontaneous yawns remains
unexplained.
Recently yawns have seen a revival of
interest due principally to the work of Robert
Provine, a neuroscientist professor of
psychology at the University of Maryland,
Baltimore [1], and of Olivier
Walusinski, a French country doctor with a wide
experience in many fi elds of medicine including
obstetrics and ultrasound. Walusinski founded
"Chasmology," a new discipline, deriving its
name from the Greek khasma, meaning opening
wide, and brought yawns to the limelight.
Besides his own publications, all kinds of
useful information and a very thorough biography
can be found on his website [2,3].
These experts are not the only ones who have
wondered why we yawn, given that as many as
nearly 20 hypotheses have been formulated. The
main theories put forward to explain the
phenomenon are briefly the following.
Respiratory and circulatory hypotheses date back
to Hippocratic times when scholars believed that
yawning removed "bad air" from the lungs and
increased oxygen circulation in the brain. A
modern derivation of these theories claims that
yawning occurs when oxygenation is low either in
the blood or brain or both. Robert Provine
dispelled once and for all the oxygenation
theory. Provine firstly noted that people do not
yawn more frequently when they exercise. By
means of experimental evidence, he then asked
his subjects to inhale air with higher than
normal levels of CO 2 and recorded that they did
not yawn. On the contrary when the subjects
inhaled unadulterated oxygen levels, spontaneous
yawning was not inhibited [4]. This
theory, however, continues to crop up from time
to time.
Another hypothesis , the arousal theory,
claims that we yawn when we feel drowsy either
before or after sleep in order to keep us awake
or help us to wake. However, again, experimental
evidence proved the theory wrong. In fact, the
contrary is true; yawning is triggered by states
of drowsiness and low vigilance
[5,6].
The state change hypothesis, put forward as
a possible explanation by Provine, is based on
the undoubted fact that multiple behavioral
state shifts are associated with yawning.
According to him yawning may facilitate or even
trigger these transitions [7,9].
The boredom, sleepiness theory simply takes
into account that we yawn when sleepy or bored.
However, people also yawn in other
circumstances, such as when waking up or being
overwhelmed by anxiety [10,11].
Nevertheless, this theory too remains
popular.
The cooling the brain hypothesis claims that
humans and animals yawn in order to lower the
temperature of the brain, thus remaining
(similarly as in the arousing theory) alert and
less vulnerable to possible perils
[12].
The social/communication hypothesis would
explain the contagious nature of yawning. When
we see someone yawning, or even think of
yawning, we yawn too. Contagious yawning, called
by ethologists behavioral imitation and by
neurologists echokinesis, would be a form of
communication and one that could reinforce group
behavior as well as our capacity for empathy
[13,14]. Empathy is gradually built up
as we grow and some never experience it.
Autistic children and adults, whose capacity to
feel empathy is frequently impaired, are immune
to contagious yawning [15,16]. The Dutch
researcher Wolter Seuntjens sees yawning as a
subconscious enticement to sex. However, he and
his co-workers admit that they are unable to
explain how we can decipher when a yawn is
produced by boredom and sleepiness or by an
invitation to sex [17].
According to the obstetrician Richard
Roberts, yawning ex utero would simply be a
useless vestige of our fetal past [18].
Early fetuses would yawn (and hiccup) during the
first trimester of pregnancy to assure patent
airway development and consequently lung
development by preventing the formation of
tracheal and bronchial webs but would then
continue to yawn and hiccup at later stages
before and after birth without these having any
function at all.
The list could continue. As Provine notes,
however, "Those expecting a single, definitive
function may probably be frustrated by the
answer - it probably has many" [1]
Why Do Fetuses Yawn?
The most probable functions of fetal yawning
will be analyzed below, after first describing
when fetuses start yawning and how they yawn.
Since the pioneering work of de Vries and
collaborators, it has been discovered that
fetuses yawn from the early fetal stages,
starting between 10 and 12 weeks [19].
Some date the fi rst yawns even earlier, at the
end of the embryonic phase. However, prior to 12
weeks, one can note just a quick (1-2 s) wide
opening of the mouth followed by an equally
quick (1 s) closure similar to "yawn" in fishes.
One wonders if this quick wide opening can be
considered a yawn or perhaps a pre-yawn or
instead a simple wide opening of the mouth.
Yawns can be observed till the end of pregnancy
[19, 20]. In interpreting fetal yawns
one always has to take into account the shape
and characteristics of the fetal thorax
Though all can recognize fetal yawns just as
they can recognize a neonatal or an adult one
and distinguish a yawn for other kinds of mouth
openings, a thorough definition of fetal yawning
taking into account all its components has not
yet been given.
de Vries describes yawns as "a prolonged
wide-opening of jaws, followed by quick closure,
often with retroflexion of the head and
sometimes elevation of the arms." Additionally,
de Vries points out that yawns are usually
non-repetitive [ 21].
Provine, expanding on the latter point,
added a more complex (postnatal) definition
based on ethology. "The yawn is highly
stereotyped, but not invariant in duration and
form. It is an excellent example of the
instinctive "fixed action pattern" of classical
animal behaviour study, or ethology. It is not a
reflex, a short duration, rapid, proportional
response to a simple stimulus. But once started
a yawn progresses with the inevitability of a
sneeze. The yawn runs its course for about six
seconds on average, but its duration can range
from about three and a half seconds to much
longer than average. There are no half- yawns,
an example of the "typical intensity" of fixed
action patterns and a reason why you cannot
stifle a yawn. Yawns come in bouts, with a high
variable inter-yawn interval of around 68
seconds. There is no relation between yawn
frequency and duration; producers of short or
long yawns do not compensate by yawning more or
less often" [7].
Recently Asim Kurjak, an obstetrician and an
expert in 3- and 4D ultrasound, defined a fetal
yawn as, "A paroxysmic cycle characterized by a
standard cascade of movements over a 5- to
10-second period. The fetal mouth, previously
closed opens widely for 4-6 sec with
simultaneous retraction of the tongue, followed
by a quick closure and usually combined with
retroflexion of the head and sometimes elevation
of the arms (pandiculation). This harmonious
sequence is markedly different from a brief
swallowing episode." And he then adds, quoting
the work of others, "Using a color Doppler
technique, it is possible to observe the flow of
amniotic fluid through the fetal mouth,
oropharynx, pharynx and trachea to the lungs"
[22].
Beyond the very initial stages, fetuses do
not stretch their arms or pandiculate when
yawning. However, the most important and
widespread misconception is that yawns
ultimately fill the lungs with swallowed
amniotic fluid. The question of whether the
amniotic fluid contributes to lung development
has been hotly debated in the past but was
proved wrong by Jost and Policard in 1948
[23]. However, despite widespread
evidence to the contrary, many still claim that
the amniotic fluid is swallowed into the lungs
and by doing this plays an essential role in
their growth.
As to other hypotheses, clearly fetuses do
not socialize, imitate, communicate and feel
empathy and boredom or are prone to sexiness.
All these feelings and behaviors only pertain to
life after birth.
The state change hypothesis, though
plausible, could apply to the last few weeks of
pregnancy only, but fetuses start yawning long
before that. On the other hand, Richard Robert's
theory, claiming that yawns (and hiccups) help
in keeping our airways patent during fetal life,
seems the most apt to explain fetal yawns
[18]. As he says, "These reflexes may be
necessary for the normal development of the
airway passages during organogenesis in the
first trimester, helping to prevent bronchial
and tracheal webs. Bronchial webs may lead to
focal adenomatous malformation of the lung.
Should a tracheal web occur, the evolutionary
fitness for that individual is 0, since the
organism would die at birth. Both reflexes would
serve to dislodge potential webs from formation
through hydraulic pressure variation…
Hence, the reason why we yawn and hiccup is
simply because we had to do it in the fi rst
trimester to assure patent airway development,
and postnatally the reflexes just keep going"
[18]. Even this hypothesis, however,
needs further expanding and clarification.
Yawns enlarge all three dimensions of the
rib cage and stretch and push down the diaphragm
greatly in a different way from that occurring
during ordinary fetal breathing motions. Fetal
breathing motions do not enlarge the rib cage in
the same way. Fetuses when yawning also greatly
enlarge the larynx and the trachea and stretch
the head backward.
Fetal yawns initially occur predominantly,
but far from exclusively, within epochs when
general movements surface, and yawns do not
intermingle with fetal breathing movements.
Fetuses generally stop moving, yawn once, and
then resume moving. Consecutive yawns are not
the rule, but they do occur. General movements
produce notable distortions of the compliant rib
cage, with consequent squeezing of the lungs.
Taking a good yawn could help the lungs to go
back into shape while redistributing lung fluid
in all its compartments. The immature lung
contains relatively few elastin and collagen
fibers and has little elastic recoil
[24].
Besides stretching the diaphragm and
enlarging the rib cage in a unique way, the
stretching of the spinal column and neck during
yawns could possibly be important for tracheal
expansion and growth.
The function of yawns, however, may not be
limited to the first trimester of pregnancy
alone. Mid-pregnancy periods of rest are short,
lasting on average 6-8 min. With advancing
gestation, rest cycles become more prolonged,
thus making rest and activity cycles clearly
defi ned by 29 weeks [25]. Yawns become
increasingly complex acts.
At 27-30 weeks, yawns also emerge just
before and after periods of rest which by then
begin to take on the characteristics of Quiet
Sleep. During Quiet Sleep, sustained breathing
motions are absent and only a shallow form of
breathing emerges; muscular tone is greatly
diminished and low; and consequently, the rib
cage (with its intercostal muscles), the
trachea, and the lungs tend to collapse. Having
few good yawns before and after Quiet Sleep
could rearrange the system and prevent tracheal
and pulmonary atelectasis defined as the
obstruction and collapse of the trachea and of
the lungs. Congenital or inborn pulmonary
atelectasis is more common in premature and weak
infants, but it can affect all infants. Eliot
Katz of the Pulmonary Department of the Harvard
Medical School says a propos of preterm infants
that sighs and yawns "Are important to
redistribute surfactant and maintain ventilation
perfusion matching. A loss in lung volume
predisposing them to the development of
peripheral airway closure is particularly
relevant to young infants. Lung volume falls if
respiration ceases as during apnoeas, which is
one of the reasons of the high respiratory rate
of infants (term and preterm). One of the main
functions of sighs in preterm infants is to
reverse falls in lung volume caused by apnoeas"
[26,28].
The problem of tracheal and lung collapse
with the possible formation of atelectasis is
well known to anesthesiologists and surgeons,
especially those involved in cardiac and
abdominal surgery. In infants, given their
highly deformable chest wall, the problem is
particularly severe and acute. The usefulness of
deep sighs and yawns could, however, extend to
later stages in life.
After surgery, even adult patients are
encouraged to take deep breaths and yawn (as
well as to cough) [29,30].
However, as Provine says, yawns may have
more than one function [1]. Yawns
preceding and following sleep may also fulfill
the function hypothesized by Provine, being
inducers or precipitators of an emerging
state.
Additionally, in the late stages of
pregnancy, yawns may have a preparatory
communicative function. After birth, yawns start
to belong increasingly to the category of facial
expressions. Infants and young children are
intent in fighting for their survival and trying
to have their basic needs met, including sleep,
a predominant feature of their lives. Yawns
clustering before epochs of rest may be
anticipatory in the sense of preparing the
future neonate to communicate albeit not
consciously the need for sleep.
The contagious and imitative component of
yawning does not appear in them before 3-4 years
of age. Possibly, no matter how exhausted and
sleepy their caregivers may be, young children
still need to be fed and put to sleep, signaling
the latter need with one or more yawns. Some
could say that infants are selfish creatures,
but being utterly dependent on their caregivers
for survival as they are, they cannot take into
account the fatigue of those looking after them
and providing for their essential demands. If
they did so, our species would have long been
extinct.
Au cours des derniers stades de la
gestation, les fonctions fœtales subissent
un changement et un développement accrus,
préparant le fœtus à la
transition vers son environnement postnatal.
Cette maturation rapide est observée pour
la respiration, la déglutition, les
fonctions sensorielles, le sommeil et de
nombreux autres processus, avec des changements
de comportement correspondants.
De 35 à 40 semaines de gestation, les
fœtus sont capables de vivre ex-utero sans
aide, mais il est de plus en plus
considéré que même les
nourrissons nés entre 35 et 36 semaines
peuvent en pâtir à long terme.
Ce livre, qui complète le volume
précédent de l'auteur sur le
développement des mouvements fœtaux
normaux pendant les 25 premières semaines
de gestation, discute en détail
l'ensemble des phénomènes
comportementaux observés pendant les 15
dernières semaines de gestation, avec une
analyse attentive de leurs relations
mutuelles.
