It’s widely assumed that yawning occurs because we are tired or bored or because we see someone else doing it, but there isn’t any hard evidence to support these beliefs. Scientists do not know all of the biological mechanisms of the yawn, but tend to agree that a yawn is an involuntary respiratory reflex, which regulates the carbon dioxide and oxygen levels in the blood. Technically, a yawn is the reflex opening of the mouth followed by the deep inhalation and slow exhalation of oxygen. The very act of yawning is but one of a number of involuntary reflexes controlled by the spinal and nerve centers the onset of a yawn is triggered either by fatigue, or by sheer boredom as, at those times, breathing is shallow, and little oxygen is carried to the lungs by the oxygen-toting cardiovascular system. Hope this answerd your question!