Latest research
awakens debate
over why people
can’t keep their
mouths closed
By Laura Sanders
Scratching relieves an itch, sneezing clears out the nose and drinking relieves thirst. Andyawning…doessomething.
tongue hunkers down, and muscles in
the face, mouth and diaphragm engage
as the head tilts back. Air streams in.
As the yawn reaches its peak, airflow
halts briefly, eyes close, and muscles
go rigid as they stretch. The long, slow
exhale allows muscles to return to their
normal positions.
Many researchers are convinced that
this complex series of movements, which
takes about six seconds on average, must
somehow affect the body.
A yawn’s obvious gulp of air led many
scientists to think that a yawn’s job was
to replenish oxygen in the brain. So far,
researchers haven’t found evidence supporting this suspicion. Yawning hasn’t
been shown to wake up a groggy body
or brain, either — a logical expectation,
since yawns strike most frequently when
a person is tired. Instead, new studies
led by Gallup suggest, a yawn may be a
thermostat, cooling an overheated brain,
a position he argues in the January
Neuroscience and Biobehavioral Revie ws.
Brain chill
After a yawn, rats experienced a temperature drop in their brains, Gallup
and his colleagues reported online last
September in Frontiers in Evolutionary
Neuroscience. Beginning about a minute
before a yawn, thermometers embedded inside the rats’ skulls measured a
brain temperature increase of about 0.1
degrees Celsius over the average temperature. Post-yawn, the temperature
dropped until it reached its starting
point.
In another set of tests, rats yawned
more as the temperature rose and their
brains heated up, Gallup and colleagues
reported in the February Ethology.
reported in the February
36.75
Cooling yawn Before a yawn, rats’ brain
temperatures rose. Post-yawn, temperatures
dropped. Yawning and stretching (and stretching alone) also showed cooling effects.
Yawning and brain temperature
36.70
Temperature (°C)
36.65
36.60
36.55
36.50
Yawn
Yawn and stretch
Stretch
36.45
– 3
–2–10123
Minutes from yawn
SOURCE: M.L. SHOUP-KNOX ET AL/FRON TIERS IN EVOLU TIONARY
NEUROSCIENCE 2010
When the outside air got too warm,
though, the rats yawned less, perhaps
because this air was too hot to be helpful. Budgies, a type of parrot, also yawn
more when it gets warmer. These results
suggest that a big yawn acts as a radiator,
bringing cooler blood from other parts of
the body up to the brain, while flushing
warmer blood down through the jugular
vein, Gallup says.
But a yawn might simply happen at
the same time as a brain temperature
adjustment, says clinical neurologist
Adrian Guggisberg of the University of
Geneva. The same brain regions could
control yawning and brain temperature
changes, so that when one occurs, so does
the other.
Whether the job of a yawn is to cool or
not, it’s still true that a warm room can
trigger a yawn. Other triggers are obvious to anyone who has suffered through
a mind-numbing meeting. Boredom,
drowsiness, hunger, stress and anxiety
may also be yawn instigators. “All these
