There is no question that yawning occurs most frequently before and after sleep, and the subjective feeling of drowsiness accompanies increased yawning. So maybe yawning helps keep us awake.

Researchers tested this hypothesis by inducing yawning in human subjects and then observing brain activity with encephalography as they yawned. The EEG produced no evidence that yawning increased vigilance in the brain or central nervous system.

Some researchers have suggested the opposite — that yawning lowers arousal and helps us go to sleep. But even though yawning and drowsiness occur together, no experiment has shown a causal connection between the two.

Could the purpose of yawning be regulation of body heat?