| 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. |