Studies on the physical and wning The act of yawning has been observed in all vertebrates, and occurs in humans as early as minutes after birth, so it must have some definitive physiological purpose. Until recently, most scientists believed yawning was a respiratory function, triggered by a surplus of carbon dioxide in the bloodstream. In the past decade, however, studies have suggested that yawning acts as a mediator of activity and arousal levels. The evaluation that follows will examine three such studies and the evidence they contribute ....