The yawn reflex (pandiculation) is not a classical neural reflex arc, but rather a coordinated neural and muscular arousal reflex mediated by the brain stem. A yawn is a sequence of events that begins with a deep inspiration of air and ends with a forced and deep expiration of air concurrent with a general contraction of several muscle groups, especially those associated with mouth, throat, and face. Yawns last four to eight seconds and the yawn reflex is well established by the second trimester of fetal development. Male yawns tend to be more vigorous than female yawns. The exact mechanisms stimulating the yawn reflex remain scientifically contentious.

Anatomically, the yawn is a coordinated movement of thoracic muscles, diaphragm, larynx, and palate. The chest wall expands, the diaphragm lowers, the palate rises and there is a tendency to move the tongue downward and to the rear of...