Research suggests that yawning provides a
brain cooling function in homeotherms, and that
excessive yawning may be a useful diagnostic
indicator of abnormal thermoregulation in
humans. Accordingly, the frequency of yawning
should increase during instances of
hyperthermia, but not fever (i.e., pyrexia),
since this represents an elevation in the
homeostatic set point rather than
thermoregulatory failure. To our knowledge, no
research has investigated the association
between yawning frequency and fever in humans.
Here we present the hypothesis that frequent
yawning could be used as an initial signal for
fever relief, either through the effectiveness
of antipyretics or the natural break of a fever.
Applications of this research include the
improved behavioral monitoring of patients.
Hypothesis
Thermoregulation in homeotherms requires
preserving a relatively constant temperature,
and deviations in this optimal range trigger a
combination of autonomic and behavioral
mechanisms controlled by the central nervous
system [1]. Recently it has been
proposed that yawning serves a thermoregulatory
function, acting to counteract intermittent
rises in brain temperature [2&endash;4].
Mechanisms by which yawning could promote brain
cooling in humans include increased cerebral
blood flow, countercurrent heat exchange, and
evaporation of sinus mucosa within the paranasal
sinuses [5].
Numerous reports have confirmed and
replicated the specific predictions derived from
this hypothesis, now referred to as the
thermoregulatory theory (for a recent review and
evaluation, see [6]). For instance,
neurophysiologic research on rats shows that
yawns are triggered during rapid increases in
brain temperature ( 0.11 C), and following the
completion of this motor pattern the previously
rising temperatures quickly begin decreasing
back to baseline levels [7]. Additional
comparative research shows that the onset of
yawning following physical stressors is
positively correlated with elevated core
temperatures in birds [8]. Furthermore,
in a case report of a patient suffering from
debilitating bouts of excessive yawning, mild
hyperthermia (37.5 C) was documented immediately
preceding the onset of yawning, and at the
conclusion of these episodes an appreciable and
consistent cooling effect was observed within
the skull (oral temperature: 0.4 C)
[9]
Alongside research depicting consistent
fluctuations in brain and/or body temperature
surrounding yawning events, a growing collection
of behavioral studies has also demonstrated a
clear association between yawning and ambient
temperature manipulation/ variation in birds and
mammals [10&endash;12]. Consistent with
models of thermoregulatory physiology
[13], yawning increases in frequency
during initial rises in ambient temperature, but
as temperatures continue to increase yawning
rates begin to diminish since the ambient air
provides the cooling utility through
countercurrent heat exchange [10,14].
Instead, more effective evaporative cooling
mechanisms are triggered as ambient temperatures
approach or exceed internal temperature
[10].
A similar, perhaps counterintuitive,
prediction of the thermoregulatory theory states
that yawning should not increase during fever
(i.e., pyrexia) [3]. Fever represents a
regulated elevation in body temperature when
concentrations of prostaglandin E2 increase in
the brain after a rise in the hypothalamic set
point [15], and it is well-established
that fever is an adaptive and essential
defensive response to infection by pathogens
[16]. Therefore, unlike thermoregulatory
failure (i.e., hyperthermia), typical cooling
responses, such as yawning, are not triggered to
counteract heightened temperatures produced by
fever. However, an increase or decrease in core
temperature from the revised set point of the
fever will stimulate normal thermoregulatory
mechanisms [17], and thus a normal
frequency of yawning could still be observed
after the fever is determined, though this has
not been empirically demonstrated.
Antipyretic medications such as
acetaminophen, aspirin and other nonsteroidal
anti-inflammatory drugs are commonly used to
treat fever by reducing the hypothalamic set
point. Most of these drugs work by inhibiting
the enzyme cyclooxygenase and reducing levels of
prostaglandin E2 within the hypothalamus, though
other mechanisms of action have also been
suggested [18]. Immediately following
the use of antipyretics, however, the deviation
from actual temperature and the new set point
should also activate thermoregulatory mechanisms
to contribute in cooling core temperatures back
to baseline. Therefore, it is hypothesized that
frequent yawning, as well as other less overt
cooling responses, could signal the initial
effectiveness of these medications. In addition,
similar to sweating, recurrent yawns may also
signal the break of a fever in the absence of
treatment. It has already been suggested that
excessive or repetitive yawning could be useful
for identifying abnormal thermoregulation
associated with certain anti-depressant
medications [19,20] or underlying
thermal dysfunction [10,21]. If this
hypothesis is correct, observational monitoring
could also be applied when treating patients
with fever where frequent yawning could be
easily recognized and recorded.
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