Yawning occurs without any effort. It is a reflex action that forces oxygen into our lungs. The body’s oxygen supply is depleted after a long period of shallow breathing, which commonly occurs when we are tired, under stress, or have been sitting still for a long time. Yawning is believed to be induced by a high level of carbon dioxide in the blood. The wide opening of the mouth followed by a slow deep breath increases the oxygen level in the blood which helps relieve the tiredness.