The Mystery of Yawning According to conventional theory, yawning takes place when people are
bored or sleepy and serves the function of increasing alertness by reversing,
through deeper breathing, the drop in blood oxygen levels that are caused by the
shallow breathing that accompanies lack of sleep or boredom.
Unfortunately, the few scientific investigations of yawning have failed to find
any connection between how often someone yawns and how much sleep they have had
or how tired they are. About the closest any research has come to supporting the
tiredness theory is to confirm that adults yawn more often on weekdays than at
weekends, and that school children yawn more frequently in their first year at
primary school than they do in kindergarten. Another
flaw of the tiredness theory is that yawning does not raise alertness or
physiological activity, as the theory would predict. When researchers measured
the heart rate, muscle tension and skin conductance of people before, during and
after yawning, they did detect some changes in skin conductance following
yawning, indicating a slight increase in physiological activity. However,
similar changes occurred when the subjects were asked simply to open their
mouths or to breathe deeply. Yawning did nothing special to their state of
physiological activity. Experiments have also cast serious doubt on the belief
that yawning is triggered by a drop in blood oxygen or a rise in blood
carbon dioxide. Volunteers were told to think about yawning while they breathed
either normal air, pure oxygen, or an air mixture with an above-normal level of
carbon dioxide. If the theory was correct, breathing air with extra carbon
dioxide should have triggered yawning, while breathing pure oxygen should have
suppressed yawning. In fact, neither condition made any difference to the
frequency of yawning, which remained constant at about 24 yawns per hour.
Another experiment demonstrated that physical exercise, which was sufficiently
vigorous to double the rate of breathing, had no effect on the frequency of
yawning. Again the implication is that yawning has little or nothing to do with
oxygen. The word "triggered" in the passage is closest in meaning to （）.