You probably cannot remember the first time you yawned, because even fetuses yawn while they are still in the womb. This means that you were yawning even before you were born. Most people yawn when they are sleepy, but scientists are not really sure how yawning helps to keep you awake, or if that is even the intended purpose behind a yawn. It has been scientifically proven that yawning is contagious, although it is possible to forcefully stifle a yawn when you feel it coming. It is as possible to force a yawn if you try hard enough. It has also been proven that yawning can be contagious between animals and humans. Some scientists think that yawning helps to cool the brain, flood it with oxygen or that is helps with developing concentration. Some scientists think that yawning helps to clean the blood. Studies have shown that the heart rate increases when someone is yawning but scientists are not sure why. Yawning may also be a way for the body to tell you to rest. The longest yawn to have ever been recorded lasted more than 6 minutes.