Psychology Research Centre, Bournemouth
University,
Talbot Campus, Poole. UK
Abstract
Yawning and its involvement in neurological
disorders has become the new scientific
conundrum. Cortisol levels are known to rise
during stress and fatigue; yawning may occur
when we are under stress or tired. However, the
link between yawning, fatigue, and cortisol has
not been fully understood. Expansion of the
Thompson Cortisol Hypothesis proposes that the
stress hormone, cortisol, is responsible for
yawning and fatigue especially in people with
incomplete innervation such as multiple
sclerosis. This informs our understanding of the
functional importance of the brain stem region
of the brain in regulating stress and
fatigue.
Hippocrates was one of the first early
observers of human behaviour and physiology.
Reporting in 400 BC, he noticed that yawning
often preceded a high body
temperature.[1] The hypothalamus was
considered to be an important brain structure in
regulating body temperature,[2] together
with the pons, medulla and midbrain acting as
relays and mediators.[3-5]
Thermoregulatory theories are thought to be
inadequate explanations of social and contagious
yawning possibly because of the involvement of
psychological mechanisms such as
empathy,[6] and the desire towards group
identity. For centuries, philosophers,
scientists, and neurologists have proposed
yawning to be a simple need to replace oxygen
deficiency in the blood.[3] Evidence
from clinical populations such as ischaemic
stroke patients suggests that a basic and vital
need to regulate other body chemicals may
indicate the involvement of the insular and
caudate in the production of the
yawn.[7] Brain-stem lesions have often
provided evidence of the link between yawning
and release of paralysed neural pathways,
particularly in the phenomenon parakinesia
brachialis oscitans where the paralysed arm can
rise involuntarily during a
yawn.[8]
Oxytocin may also be associated with
yawning,[9,10] especially because it is
even seen in humans at pre-term.[11,12]
Depressed mothers may be more prone to cease
breastfeeding early. This may potentially lead
to damage of the stress response within the
infant as they become flooded with the cortisol
stress-response hormone.[13] As the
body's natural defence against the effects of
stress,[14] cortisol is released by the
pituitary gland to maintain the circadian rhythm
in a role within the
hypothalamus-pituitary-adrenal (HPA)
axis.[15] It is not surprising that
sleep deprivation and fatigue may be associated
with lowered cortisol levels.[16] Hence,
cortisol has a potentially significant role in
the rejuvenation and well-being of the human
body. When incomplete innervation occurs as in
multiple sclerosis (MS), excessive yawning is
frequently found with fatigue. The Thompson
Cortisol Hypothesis[17] proposes that
rises in cortisol levels are associated with
yawning. An increase in electro-muscular
activity around the jaw line occurs during
yawning, seen as a 'yawning envelope' in the
electromyogram,[18] and with elevated
cortisol levels. Threshold level of cortisol may
be required for yawning status. Functional
interactivity was significantly reinforced
during a phasic alertness task in a study at
Amiens University Hospital and the University of
Picardy Jules Verne.[19] Preferentially
activity was seen in the dorsolateral prefrontal
cortex (DLPFC) region of the brain, as compared
to during intrinsic alertness. The DLPFC region
may be involved in maintaining a state of
alertness and in temporal preparation during
alertness tasks. These teams are examining
alertness and fatigue together with cortisol
rises in healthy and MS patients both in France
and in the UK (at Bournemouth University) in a
collaborative study.
It is proposed that when the critical
threshold level of cortisol is reached because
of fatigue, yawning is elicited. This action
gives rise to increased electro-myographical
activity in the jaw muscles and regulates the
production of adrenaline from the adrenal
glands. Feedback further regulates cortisol and
adrenaline production within the HPA axis.
Discussion
There are several interesting findings of
this study, which are consistent with the
Thompson Cortisol Hypothesis. Significant
difference in saliva cortisol levels for those
who yawned, between sample one and sample two,
were found, which lends support for the
hypothesis. No significant difference was found
for the non-yawners between saliva cortisol
sample 1 and sample 2. EMG activity also
increased with elevated cortisol levels and when
yawning. Small (non-significant) rises in saliva
cortisol levels in the non-yawners (between rest
and post-stimuli) may be explained in terms of
the experimental procedure. Since two time
points of saliva cortisol sampling were taken
for both groups, it is possible that cortisol
levels rose for both groups in the presence of
yawn-stimuli but for the yawners, cortisol
levels reached the threshold necessary for the
elicitation of a yawning response. It is
understood that cortisol acts to protect our
body against stress and plays a role in the
regulation and balance of hormones released
within the Hypothalamus-Pituitary-Adrenal axis.
The yawn response may give rise to an increase
in cortisol levels to provide symptom relief
such as in lowering brain temperature as
proposed by Gallup.[2]
This may be the mechanism involved in the
excessive yawning of people with multiple
sclerosis via the hypothalamus as temperature
regulator. Fatigue as a common symptom of
multiple sclerosis seems also to be intimately
linked with yawning and elevation in cortisol
levels,[28] and it is proposed that the
setting for the yawning response in social
encounters may also involve the component of
induced fatigue, as suggested by co-workers of
the Anglo-French collaborative study into
cortisol in MS.[19]
Conclusions
Yawning and cortisol is of interest to
clinical scientists, practitioners, neurologists
and neuroscientists. Whilst still presenting a
scientific conundrum, it has presented as a
fascination for centuries but is now emerging
with potential clinical and neuro-scientific
importance, especially in the domain of
diagnostic biomarkers. Clearly, further research
is indicated; however, it is fitting that this
ancient mechanism, common to most of us that has
been reported for many centuries, is perhaps a
breakthrough for modern neuroscience and
rehabilitation.
