Yawning involves opening the mouth involuntarily while taking a long, deep breath of air. This is usually done as a result of drowsiness or weariness. Excessive yawning is yawning that happens more often than would be expected, even if drowsiness or weariness is present.
Yawning is a normal response to fatigue and drowsiness, but excessive yawning can be caused by a vasovagal reaction. This reaction is caused by the action of a nerve, called the vagus nerve, on the blood vessels. It may indicate a heart problem.
Normal yawning may happen when someone else yawns.