It is a very natural response to being tired. In fact, yawning is usually triggered by sleepiness or fatigue. ... Certain conditions can cause a vasovagal reaction, which results in excessive yawning. During a vasovagal reaction, there is increased activity in the vagus nerve.

Everyone yawns, yet the purpose of this body function is still somewhat of a mystery. People commonly yawn when they are tired or bored -- and when they see other people yawn. Excessive yawning, on the other hand, is not normal and can be a sign of a medical problem.

If the yawning is frequent and continuous, and not clearly linked to inadequate sleep, see your doctor as soon as possible. Rarely, excessive yawning is associated with a medical emergency, so if you also have symptoms such as fainting, dizziness, chest pain, shortness of breath or weakness, seek immediate medical attention.