Yawns wake up the brain

Although we associate yawning with sleepiness and boredom, its function appears to be to wake us up. Yawning expands our pharynx and larynx, allowing large amounts of air to pass into our lungs; oxygen then enters our blood, making us more alert.

Many vertebrates do it, including all mammals and perhaps birds. It also has been observed in human foetuses after just 12 weeks of gestation. In non human primates, it is associated with tense situations and potential threats.