Donald Hebb states it as follows: "Let us assume that the persistence or repetition of a reverberatory activity (or "trace") tends to induce lasting cellular changes that add to its stability.… When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in...

According to wikipedia, the jury is still out on the function a yawn serves. The article referenced above however writes to say Yawning most often occurs in adults immediately before and after sleep, during tedious activities and as a result of its infectious quality. It is commonly associat...