Hypothesis

Physiology

(1)

Respiratory/circulatory hypotheses [1]

Not favored because high CO₂ or physical exercise did not produce more yawns [1] [2] .

(2)

Sleepiness hypothesis [1]

Yawns often occur around sleep without producing EEG arousal [1] [3] [4] .

(3)

Brain cooling thermoregulation [5] [6]

Brain cooling via paranasal sinuses [7] can implicate that yawns regulate the neurovascular balance in head.

(4)

Decrease of cortisol level [8]

This may be the oversimplified one of the related consequences of yawns against stress/tiredness.

(5)

Social/communicative functions [1]

Irrelevant to human yawns.