The existence of yawning
across vertebrate species suggests important
basic functions, and the spontaneous and
involuntary nature of a yawn lends support for
it having adaptive significance. Recent research
suggests the biological function of yawning
among homeotherms is central thermoregulation.
Comparative research from birds, rats, and
humans shows that yawning reduces brain and body
temperature, is influenced by the range and
direction of ambient temperature change, and is
inhibited by methods of behavioral cooling. This
research provides strong support for the view
that yawning stimulates or facilitates cortical
arousal, demonstrating that a yawn is a
behavioral response to transient brain
hyperthermia, acting to counter intermittent
increases in brain temperature and promote
thermal homeostasis. This theory is powerful
because it not only integrates much seemingly
diverse information about yawning, but it can
also be used to generate numerous testable
predictions. Applications from this research
range from basic physiological understanding, to
the improved treatment and understanding of
diseases associated with thermoregulatory
dysfunction.
Yawning is a widespread behavioral response
expressed in all classes of vertebrates. There
is however, little agreement on its biological
significance. One current hypothesis states that
yawning serves as a thermoregulatory mechanism
that occurs in response to increases in brain
and/or body temperature. The physiological
consequences of yawning are concordant with
those needed to effectively cool the brain, such
as increases in peripheral and cerebral blood
flow.
The "brain cooling hypothesis" states that
yawning serves to keep the brain in thermal
homeostasis, and that yawning serves to maintain
optimal mental efficiency. According to this
view, yawning functions as a compensatory
cooling mechanism when other mechanisms fail to
operate favorably. A growing body of medical and
physiological evidence shows that excessive
yawning is symptomatic of conditions that
increase brain and/or body temperature (e.g.,
multiple sclerosis, epilepsy). Likewise, drugs
that increase brain temperature (e.g., certain
serotonin reuptake inhibitors) frequently
produce excessive yawning, while drugs that
produce hypothermia (e.g., opioids) inhibit
yawning.
This research suggests the existence of an
important connection between yawning and
thermoregulation, which has been heretofore been
overlooked or ignored by modern and traditional
theorists. This model provides a unifying theory
regarding the proximate mechanism of yawning in
endotherms, offering numerous testable
hypotheses.
