We review a growing body of medical and
physiological evidence indicating that yawning
may be a thermoregulatory mechanism, providing
compensatory cooling when other provisions fail
to operate favorably. Conditions such as
multiple sclerosis, migraine headaches,
epilepsy, stress and anxiety, and schizophrenia
have all be linked to thermoregulatory
dysfunction and are often associated with
instances of atypical yawning. Excessive yawning
appears to be symptomatic of conditions that
increase brain and/or core temperature, such as
central nervous system damage, sleep deprivation
and specific serotonin reuptake inhibitors.
Yawning is also associated with drowsiness, and
subjective ratings of sleepiness are correlated
with increases in body temperature. This view of
yawning has widespread application for the basic
physiological understanding of
thermoregulationas well as for the
improved diagnosis and treatment of diseases
associated with abnormal thermoregulation.
1. Introduction
Yawning is characterized by a large gaping
of the mouth and eye closure, accompanied by a
deep inhalation of air, and shorter expira- tion
[1]. A yawn can occur without gaping
[2,3], but gaping of the mouth during
the peak of a yawn is essential for increases in
facial blood flow that in turn alter cerebral
blood flow as a result [4]. This may be
why participants who are asked to clench their
teeth while yawning report the yawns as abnormal
and less satisfying [3]. Yawning is
widespread and has been observed among most
classes of vertebrates [5].
In humans, yawning occurs before birth and
as early as 20 weeks after conception
[6]. In a recent study investigating the
frequency of yawning in preterm infants,
researchers found a marked decrease in daily
yawns between 31 and 40 weeks of age
[7], and attribute this to the
development of circadian and homeostatic control
of sleep and waking. After birth and
development, yawning occurs throughout life in a
consistent fashion for most adults. It is well
docu- mented that yawning often occurs during
the first hour after waking and the last hour
before sleeping [7-9].
Among humans there is no evidence for sex
differences in the incidence of yawning. In some
primates, however, males are more prone to show
threat yawns that display their enlarged canine
teeth [10,11]. These yawns may
functionally differ from normal yawns, and can
be distinguished from such because the eyes do
not close. During a threat yawn, the individual
keeps their eyes open during the peak of the
yawn in order to monitor the effect of the
threat on the target. Yawning has long been
thought to be a sign of boredom and is commonly
interpreted as disrespectful when done in the
presence of others. Yawning is also contagious.
Seeing, hearing, reading, or even thinking about
yawning can trigger yawns, and attempts to
shield a yawn do not prevent its contagion
[12]. Under laboratory conditions,
40-50% of college students yawn in response to
seeing videotapes of people yawning
[2,3,13]. But people are less likely to
yawn when they suspect they are being observed
by scientists [14]. Individual differ-
ences in susceptibility to contagious yawning
have been shown to be related to differences in
empathic ability and self-processing
[13].
Witnessing people yawn has also been shown
to activate parts of the brain associated with
self-processing [15]. Yawning can occur
as a consequence of a variety of interactions
among neurotransmitters and neuropeptides in the
brain [16], in- cluding dopamine,
excitatory amino acids, acetylcholine,
serotonin, nitric acid, adrenocorticotropic
hormone-related peptides and oxyto- cin. In
contrast, yawning is inhibited by opioid
peptides [16]. While the neurochemical
mechanisms underlying yawning are not completely
understood, the ability to induce yawning using
drugs under labo- ratory conditions has proven
to be a valuable research tool. Changes in
yawning can be an important factor in our
understanding of the physiopathology of certain
diseases and action of new drugs, parti- cularly
those that have dopamine involvement
[17].
Attempts to identify the
adaptive/functional/biological significance of
yawning (reviewed by Smith, 1999) have lead to
little consensus [12,18]. Theories about
yawning range from increasing alertness
[8,14], an expression of boredom,
unconcern, or indifference [14,19], to
aiding in the removal of potentially infectious
substances from the tonsils [20].
Yawning has also been thought to be an indicator
of hemorrhage [21], motion sickness
[22], and encephalitis [23]. A
commonly held view is that yawning functions to
equilibrate CO2 and/or O2 levels in the blood
[24]. A spin off of this theory is that
yawning functions to correct imbalances in
cerebral oxidative metabolism [25].
Contrary to public opinion, however, having
subjects breathe increased levels of oxygen or
carbon dioxide leaves yawning unaffected
[26]. But despite this evidence some
researchers continue to look at yawning as a
response to localized hypoxia in the brain
[27]. Olivier Walusinski maintains an
open-access, growing collection of selected
current as well as historical research on
yawning [28].
2. Yawning as a brain cooling mechanism
The brain is a metabolically expensive organ
second only to the gut, responsible for about
16% of our total energy consumption
[29]. Brain temperature in humans is
determined by a number of variables, including
the temperature of arterial blood going to the
brain, rate of blood flow, and rate of metabolic
heat production [30]. There are specific
chemical and thermoregulatory mechanisms that
operate to maintain optimal brain temperature
[31,32].
Based on fossil evidence of brain expansion
in humans over the past several million years,
it has recently been determined that as much 52%
of the variance in cranial capacity can be
accounted for by global cooling [33].
Consistent with this finding, Falk (1990) has
championed the view that brains can only grow as
large as they can be effectively cooled; known
as the radiator hypothesis [31]. The
radiator theory posits that emissary veins grew
in tandem to support the cooling needs of larger
brains during human evolution. In other words,
as brains evolved and grew larger, there had to
be corollary adaptations that increased venous
cooling.
We recently advanced the hypothesis that
yawning may function as an evolved brain cooling
mechanism [2]. The physiological
consequences of yawning are analogous to those
needed to effectively cool the brain, such as
increases in peripheral and cerebral blood flow
[4,34-36]. In addition, there is growing
evidence that yawning occurs before, after, and
during instances of abnormal thermoregulation,
heat stress, and hyperthermia.
According to this hypothesis yawning serves
as a compensatory cooling mechanism that
functions to maintain optimal levels of mental
efficiency. There are two forms of selective
brain cooling (SBC) that operate in humans:
precooling of arterial blood destined to the
brain, with cool venous blood returning from the
nose and forehead, and the use of venous blood
to cool the brain directly [37].
Specifically, SBC involves a reduction in brain
temperature below that of arterial blood from
the trunk. In order to test the hypothesis that
yawning may be a SBC mechanism, we manipulated
nasal breathing and forehead temperature in
participants watching videotapes of people
yawning. Nasal breathing operates by precooling
arterial blood destined to the brain with cooler
venous blood that drains from the extracranial
evaporative surfaces [38], while direct
cooling of the forehead and facial emissary
veins simulates a combination of both cooling
mechanisms [39].
Results showed that nasal breathing and
forehead cooling virtually eradicated contagious
yawning in college students [2].
Although previous research has reported the
apparent impossibility of "nose yawns"‚
[9,26], participants in this study were
not instructed to close their mouths or keep
their mouths closed and were therefore free to
yawn at any time.
There is considerable evidence that nasal
breathing and forehead cooling are specific
brain cooling mechanisms [38-41]. The
vertebral venous plexus, located in the
brainstem, is cooled by the vertebral artery as
a result of nasal breathing [38]. Nasal
breathing has also been shown to cool other
parts of the brain including the frontal cortex
[40]. Nasal mucosal blood flow decreases
in response to skin cooling, in- creases in
response to skin warming, and rises in response
to increases in core temperature [42].
Cooling the forehead has been shown to cool
blood in the emissary, diploic, ophthalmic, and
facial veins which is transferred via the dural
venous sinuses to the dura mater
[39].
If yawning serves as a compensatory cooling
mechanism, one would expect yawning to be
related to ambient temperature. As ambient
temperature rises it becomes increasingly more
difficult to maintain optimal thermal
homeostasis, and therefore one would expect
yawning to occur more frequently. This
prediction was tested recently in parakeets
(Melopsittacus undulatus) [43] which
were chosen due to their large relative brain
size [44]. As ambient temperature
increased there was a significant increase in
the incidence of yawning. In addition, yawning
became progressively more frequent as ambient
temperature rose to within 5°C of parakeet
body temperature, indicating a breech in thermal
homeostasis occurring at or near this point
[43].
2.1. The brain cooling hypothesis
Body temperature is a balance between heat
production and heat dissipation. By lowering
brain temperature slightly under conditions of
thermal stress to maintain thermal homeostasis,
our model predicts that mental efficiency and
vigilance should be raised as a result. Yawning
increases blood flow and acts like a radiator by
removing hyperthermic blood from specific areas
while at the same time introducing cooler blood
from the lungs and extremities.
During exercise-induced heat stress
(hyperthermia), blood flow is increased from the
surface of the head into the cranial cavity
[45]. Yawning causes an increase in both
blood pressure [34] and heart rate
[36,46], and the constriction and
relaxation of facial muscles during ayawn also
increase facial blood flow which in turn may
alter cerebral blood flow [4]. The
increase in facial blood flow due to yawning
aids in the dissipation of heat through emissary
veins. In addition, increases in arousal as
measured by skin conductance occur during
yawning [35], and this may promote
further cooling through vasodialation. Similar
physiological adjustments occur during powerful
stretching, and yawning is often accompanied by
stretching [9]. The gaping of the mouth
and deep inhalation of cool air taken into the
lungs during a yawn can also alter the
temperature of the blood going from the lungs to
the brain through convection. Tearing of the
eyes that some people experience at the peak of
a yawn may likewise play a role in dissi- pating
heat from the skull. Not only does nasal
breathing and forehead cooling block yawning, we
have noticed that a deep nasal inhalation can
extinguish a pending need to yawn.
2.2. Further predictions of the model
According to the brain cooling hypothesis
there ought to be as yet unidentified mechanisms
that serve to constrain yawning to a relatively
narrow range of ambient temperatures, or what
can be referred to as a thermal window
[2]. The model predicts that yawning
should increase in frequency as ambient
temperature approaches body temperature
[2,43], but should not occur when
ambient temperatures reach or exceed body
temperature (37°C in humans) because that
would serve to send warm rather than cool blood
to the brain. Likewise when temperatures fall
below a certain point, perhaps ‚
-10°C, yawning would cease to be adaptive
because sending a wave of unusually cool blood
to the brain could cause a thermal shock.
Another counterintuitive but testable
implication of the model is that yawning should
diminish during fever. An elevation of body
temperature can occur either due to
thermoregulatory failure (hyperthermia), or from
intact homeostatic responses such as fever
[47]. It is well-established that fever
is an adaptive host defense mechanism and an
essential defensive response to infection by
pathogens [48]. Consistent with this
notion Mariak et al. (1998) found that brain
temperature during fever is not selectively
suppressed by any specific thermolytic
mechanisms [49], and research has shown
that attempts to treat fever can have harmful
effects on critically ill patients, leading to
an increase in mortality [50,51].
Therefore, the mechanisms that trigger an
increase in the thermoregulatory set point
(fever) in the hypothalamus may, as a testable
implication of our model, override or turn off
normal operating thermal mechanisms such as
yawning (also in the hypothalamus) to fight the
infection. An alternative possibility is that
fever simulates an increase in ambient
temperature and re- duces the likelihood of
yawning for the reasons mentioned
previously.
3. Yawning and abnormal thermoregulation
3.1. Multiple sclerosis
Multiple sclerosis (MS), an inflammatory,
demyelinating disease of the central nervous
system has been linked to thermoregulatory
dysfunction [52-54]. An underlying
mechanism involved in the enhanced sensitivity
to changes in temperature among MS patients is
the direct influence on both the sodium channels
and current necessary for polarization of the
axon [55]. Increases in temperature
diminish the depolarizing current, while
decreases in temperature (cooling) have the
opposite effect. An additional symptom of MS is
sweat gland function impairment [56].
Blood from the face and forehead is cooled by
the evaporation of sweat from the surface of the
skin [39,40,45]. Therefore, impaired
sweat gland function has detri- mental effects
on the ability to dissipate heat, and this has
proven to be fatal for some MS patients who
develop severe hyperthermia during hot baths
[57]. Consistent with the view that
yawning is involved in thermoregula- tion,
excessive yawning is a common symptom of MS
[58], and some MS patients experience
temporary relief of symptoms following a yawn.
Changes in temperature can have significant
effects on the severity of MS symptoms; daytime
heating from the sun's radiation can worsen the
symptoms, while a cold shower can relieve the
symptoms [55,59]. Head and neck cooling
have proven particularly effective for
alleviating MS symptoms [60]. Just as
applying cold packs to the forehead blocks
yawning, cooling of similar parts of the body
relieves MS symptoms.
3.2. Epilepsy
Epilepsy is a seizure disorder caused by
abnormal electrical discharges from neurons in
the cerebral cortex. Frequent and repetitive
yawning is associated with some forms of
epilepsy [17,61-63], and the feeling or
act of yawning may also be a part of the
epileptic seizures themselves. Recent data show
that some epileptic patients notice yawning
before and after seizures [63]. In
addition, 50% of these patients reported that
yawning made them feel at least "slightly
better" [63]. Yawning has also been
described as a possible aura of epileptic
seizure [64], and has even been observed
during infantile spasms [65].
Epilepsy has been linked to
thermoregulation, and exposure to hot water or
increased temperature can aggravate epileptic
symptoms [66- 68]. Hot water epilepsy is
a form of reflex epilepsy, where a seizure is
precipitated by an external sensory stimulus
[69]. In this form of epilepsy,
immersion in hot water (often due to bathing)
can promote seizures [67]. A substantial
number of patients find these seizures
pleasurable and a significant proportion attempt
to trigger these heat-induced seizures
themselves [69]. In a parallel fashion
normal individuals often feel a sense of
pleasure/gratification during a yawn
[3], and this has been attributed to the
effect that yawning has on dopaminergic activity
in the central regions of the brain
[17]. Further research could investigate
the possible connection between seizure activity
and dopamine release.
Circadian thermoregulation is disrupted in
limbic epileptic rats and corresponds to
regional hypothalamic neuronal loss
[70], with the highest seizure frequency
occurring after the circadian temperature peak.
An infrared study investigating facial
temperature has shown that mesial temporal lobe
seizures may activate (or be activated by)
thermoregulatory mechanisms in humans
[71], and temperature regulation has
been documented in focal epilepsy as well
[72]. This is consistent with reports of
yawning and/or the feeling of a need to yawn
during epileptic seizures
[63‚65].
3.3. Headaches
A migraine prodrome is a constellation of
symptoms occurring before a migraine headache.
Yawning is a common precursor to a migraine
headache [73,74], and can enable the
individual to anticipate and predict the attack,
and excessive yawning has been reported as a
migraine premonitory symptom [73]. In a
study that investigated the signs and symptoms
before, during, and after migraines over a 3
month period, yawning was one of the best
predictors of the onset of headache
[75]. Blau (1991) also reports that
yawns are common after an attack
[76].
Headaches, including migraines and cluster
headaches have been consistently linked to
direct or indirect increases in brain and/or
core temperature [77-79]. Kelman (2007)
studied over 1200 migraine pa- tient evaluations
for possible triggers to the onset of an acute
migraine [78]. Over 75% of the patients
reported being conscious of specific triggers to
an attack. Of the triggers, stress was listed
most frequently (nearly 80%), while sleep
disturbance (50%), heat (30%), and exercise
(22%) were also mentioned; all of which can have
a significant impact on core and brain
temperature. Consistent with this
interpretation, Gomershall and Stuart (1973)
found that migraine headaches signi- ficantly
increase on days when there are 2 or more hours
of sunshine [80].
During cluster headaches it has been
reported that there is an increase in medial
forehead sweating [81]. Peres et al.
(2000) also describes how increased body heat
might precipitate cluster headache attacks by
the alteration of melatonin concentrations
[79]. In one study, exercise, a hot
bath, or elevated environmental temperature
provoked cluster headaches within an hour in 75
out of 200 patients [77]. Further
evidence shows that heat exhaustion is often
accom- panied by dizziness and headache
[82,83]. Elevated indoor air temperature
can also increase headache intensity
[84]. Specific cases have been reported
where hot baths or even pouring hot water over
one's head can lead to headaches
[85,86].
3.4. Stress and anxiety
As an extension of the brain cooling
hypothesis, it seems reasonable to suppose that
propensity to yawn should increase while people
are engaged in difficult mental tasks
[2]. At the metabolic level, heightened
mental activity would be expected to promote an
increase in brain temperature and eventually
trigger compensatory yawning. Recent research
shows that decreases in hippocampal glucose are
associated with cognitively demanding tasks in
rats [87]. Glucose metabolism due to
mental processes supports the argument that
increases in cognitive load would be expected to
increase localized brain temperature. Con-
ditions that promote stress and anxiety may work
in a similar fashion.
Being under stress has been shown to be
conducive of yawning in humans [88]. In
rats, yawning and grooming often occur during
mild stressors such as shocks [89].
Interestingly, animals respond differently to
constant vs. intermittent stressors
[90], with intermittent stressors (foot
shock and swimming) increasing the frequency of
yawning while constant stressors decreased
yawning. Yawning has also been shown to be a
common symptom of a variety of stressors and
measures of anxiety in primates. Displacement
activities (including yawning) tend to occur in
non-human primates during psychosocial stress
[91]. To list a few, yawning has been
used as an indicator of anxiety in chimpanzees
[92] and olive baboons [93].
Castle et al. (1999) report that yawning
increased by 40% in female olive baboons when
the nearest animal was a dominant individual
opposed to a subordinate [93].
Stress raises core body temperature and can
lead to stress-induced hyperthermia
[94], and direct evidence of
stress-induced hyperther- mia has been widely
observed in mice [95-97]. The fact that
stress has been shown to produce both increases
in core body temperature and increases in
yawning frequency is consistent with the idea
that yawn- ing may be a thermoregulatory
mechanism.
3.5. Schizophrenia
Clinical observations reveal that
schizophrenics yawn less [25]. In
addition, schizotypal personality traits, as a
measure of premorbid schizophrenic tendencies
have also been shown to diminish the propensity
to show contagious yawning among college
students [13,15]. Abnormal
thermoregulation is common among individuals
suffering from schizophrenia [98-100].
Dysregulation of body temperature among
schizophrenics includes different baseline
temperatures, abnormal daily range in
temperature, diurnal variation with an earlier
peak, impaired ability to adapt to heat stress,
and the ability to compensate more effectively
to cold stress. It is well-established that
schizophrenics have a significantly lower
metabolism and are at a higher risk for
metabolic syndrome [101]. Consistent
with the brain cool ing hypothesi s, Lehmann
(1979) argues that the reduced incidence of
yawning in schizophrenics may be a result of
reductions in brain metabolism (and by
implication brain temperature)
[25].
3.6. Serotonin
Selective Serotonin Reuptake Inhibitors
(SSRIs) are one of three main groups of
antidepressants, commonly used in the treatment
of depression and anxiety. SSRIs are designed to
allow serotonin in the brain to be utilized more
efficiently. Inhibition of serotonin reuptake by
the presynaptic neuron is thought to increase
the level of available serotonin that can bind
to postsynaptic receptors. A common and well
documented side-effect of SSRI therapy is
excessive yawning [102,103]. The use of
paroxetine, another SSRI, has also recently been
shown to produce excessive yawning
[104,105].
Serotonin is a vasoactive compound that
regulates skin blood flow, which is a major
mechanism in thermoregulation [106].
Increases in serotonin have been linked to
increases in brain and core temperatures
[107,108], and there is also evidence
that increases in serotonin inhibit the ability
to effectively deal with heat stress
[109]. During exercise, individuals
using SSRIs exhibit higher body temperatures
than those taking placebos. Night sweats have
also been reported by women taking SSRIs for
treatment of depression [110]. The
evidence suggests that excessive yawning in
patients taking SSRIs may be a consequence of
increases in brain and core temperature produced
by these drugs.
3.7. Opioids
Opioid receptor agonists (kappa receptors)
produce a hypothermic response in rats
[111], and consistent with the
possibility that yawning may be a
thermoregulatory mechanism, opioid peptides
inhibit yawn- ing [16]. Also consistent
with this proposition, naloxone, an opiate
antagonist used to counter the effects of opioid
overdose, has been shown to produce excessive
yawning [112]. Further study could in-
vestigate the incidence of yawning in response
to a variety of different drug treatments as
means of screening drugs for possible abnormal
thermoregulatory side effects.
3.8. Head trauma/stroke
Excessive yawning has also been implicated
as a symptom in pa- tients with various head
injuries, such as brain stem ischaemia
[113], and cardiac conditions, such as
cardiac tamponade [114]. Hyperthermia is
a common consequence of central nervous system
injury [115], and brain temperature
often exceeds systemic temperature in
brain-injured patients [116-118].
Furthermore, it has been found that
postoperative hyperthermia after cardiac surgery
can produce cognitive dysfunction
[119-121]. These problems have recently
led to closer monitoring and attempts to control
brain temperature in surgical patients with head
trauma/stroke [115,118,122,123].
3.9. Hypothalamus
Among its many functions, the hypothalamus
operates to regulate body temperature. Core
temperature is maintained by a variety of
thermoregulatory responses, all of which are
largely controlled by the hypothalamus
[124-126]. Research based on tissue
slices has pin- pointed the circuits within the
hypothalamus serving thermoregulation
[127]. Consistent with the idea that
yawning is a thermoregulatory mechanism, yawning
is also strongly influenced by the hypothalamus.
Yawning is under the control of many
neurotransmitter and neuropep- tides, and the
interaction of these substances in the
paraventricular nucleus (PVN) of the
hypothalamus has been shown to affect yawning
[16]. For instance, injecting
neuropeptide orexin-A into the PVN elicits a
cortical arousal response followed by yawning in
rats [128]. Consistent with our
hypothesis, orexin-A has also been shown to
induce hyperthermic reactions in rats,
increasing colonic [129] and abdominal
temperatures [130].
3.10. Sleep
Yawning is commonly associated with being
tired or sleepy. Con- trary to popular belief,
however, yawning is not correlated with sleep
duration [131]. Also, yawning does not
occur during sleep [132], which is
consistent with the idea that the need for
optimal mental processing diminishes while
asleep. Frequent instances of yawning tend to
occur shortly after awakening as well as before
going to sleep [8,9,131].
Thermoregulation and sleep are interrelated
[133-136]. Research shows that the
interaction between thermoregulation and sleep
occurs at the level of the preoptic-hypothalamic
thermostat [136]. The onset of sleep
initiates a decline in the core body temperature
curve [137,138]. This reduction in core
temperature may be due to a relaxed state that
leads to a significant decrease in cutaneous
sympathetic nervous system activity, which in
turn facilitates enhanced peripheral blood flow
[139-141].
It has been argued that temperature changes
before and after sleep act in a positive
feedback loop, and core body temperature and
sleep vary inversely [133]. Consistent
with this interpretation, Kumar (2004) presents
evidence that the circadian modulation of body
tem- perature alters sleep propensity
[135]. Increases in the tendency to
wake-up occur in tandem with increases in core
body temperature in the morning [142].
Yawning in early morning may therefore be a
compensatory thermal stabilizing mechanism.
Waking also disrupts a relaxed state, and rapid
increases in locomotor activity, as well as in
conscious brain activity and the ensuing
metabolic increases in tem- perature may trigger
yawning.
Prolonged sleep deprivation in rats has been
shown to increase deep brain temperature
[143]. Subjective ratings of sleepiness
in hu- mans correlate with increases in skin
temperature while lying down [144] and
with increases in core body temperature when
standing. In addition, hot water consumption has
been shown to increase body temperature as well
as sleepiness [145]. Thus, it appears
that variation in body temperature is associated
with corresponding variation in sleepiness.
Consistent with the idea that yawning is
triggered by an increased brain metabolic load
it is common for the onset of a yawn to prompt
people to comment on how tired they feel.
4. Discussion
Evidence from diverse sources is consistent
with the idea that yawning may be a
thermoregulatory mechanism (see Table 1).
Multiple sclerosis, epilepsy, schizophrenia,
treatment for opiate withdrawal, sleep
deprivation, migraine headaches, stress and
anxiety, central ner- vous system damage, and
serotonin have all been linked to temperature
regulation/dysfunction, and each of these
conditions have been shown to affect yawning.
Likewise, drugs that increase brain temperature
(e.g., certain serotonin reuptake inhibitors)
frequently produce excessive yawning, while
drugs that lead to hypothermia (e.g., opioids)
inhibit yawning. Being tired and/or bored is
commonly associated with yawning, but according
to the brain cooling hypothesis yawning func-
tions to antagonize rather than promote sleep
[2]. We believe that much like the
experience of feeling tired, boredom may also be
a byproduct of an increase in brain and/or core
body temperature.
Among any number of testable predictions
that follow from this hypothesis is that when
attention and cognitive processing begin to
fade, as a result of rising brain temperature
due to an increased meta- bolic load, yawning
would be expected to increase mental efficiency
and vigilance by reinstating optimal brain
temperature. Contrary to this idea, a recent
study has shown that electroenchaplographic
changes (EEG) indicate no arousing effect after
yawning [46]. However, it is possible
that yawning increases mental efficiency and
vigilance in ways that cannot be accurately
detected by EEG. Hoagland (1936, 1938, 1949)
found that increasing body temperature resulted
in faster alpha rhythms [146-148], and
in accord with yawning involvement in ther-
moregulation, Guggisberg et al. (2007) report a
significant slowing in alpha rhythms after
yawning [46]. Furthermore, Guggisberg et
al. (2007) found an increase in heart rate
variability after the onset of yawning
[46], which is also consistent with the
dissipation of heat from the emissary veins as a
result of increases in blood flow and heart
rate.
The present review outlines a novel approach
and interpretation of the functional
significance of yawning. The value of this
research extends beyond the study of yawning per
se, as theoretical and em- pirical findings
appear to have direct implications to many
facets of human physiology and behavior. The
integration of diverse fields and disciplines
including medicine, disease, and abnormal
thermoregula- tion is consistent with this new
view of yawning.
While studies suggest that temperature
regulation is controlled by a hierarchy of
neural structures, recent evidence indicates
that the mechanism that triggers yawning may lie
within this foundation. The research presented
here suggests the existence of an important con-
nection between yawning and thermoregulation,
which has heretofore been overlooked or ignored
by modern and traditional theorists. Further
research is essential however, before we can
completely understand this relationship. But the
applications of this research are nonetheless
intriguing; ranging from basic physiological
understanding, to improved treatment and
understanding of diseases such as multiple
sclerosis and epilepsy, to using symptoms of
excessive yawning as a diagnostic tool for
identifying instances of thermoregulatory
dysfunction.
