Yawning is a stereotyped behaviour present in most mammals from rodents to humans and has been described since antiquity. Hippocrates considered yawning to be an exhaustion of the fumes preceding fever. Modern medicine did not pay much attention to it until the 1980s, when, with advances in neuropharmacology, yawning proved to be a valuable tool for the assessing dopaminergic activity and the pharmacological properties of new drugs. However, its precise role in human physiology is still unknown and its mechanisms remain unclear.
Yawning can be divided into 3 distinct phases: a long inspiratory phase, a brief acme and a rapid expiration, frequently but not always associated with stretching, tears, shivering, obstruction of the eustachian canal (causing a reduction in audiologic acuity), followed with a feeling of comfort. The average duration of the yawn is 5 s, (range, 3 to 45 s). The earliest appearance of yawning was observed in a 15-week-old embryo.