Why do we yawn when we're tired?

Ah, a most intriguing question, one that has baffled many a thinker throughout the ages. Indeed, sir, yawning is often a telltale sign of fatigue, and it seems our bodies have a rather clever way of telling us when we need rest. One theory suggests that yawning helps to increase alertness by stimulating the brain with more oxygen, quite a useful function when one has been slumped over a desk or staring blankly at a screen for far too long. Another rather charming idea is that yawning may be contagious, and this may have evolutionary roots, helping groups of early humans to synchronise their states of alertness. Whether it's a sign of tiredness, a way to stay awake, or simply an act of solidarity, one cannot deny the sheer universality of the yawn.