Functions of Spontaneous Yawning: A Review

Abstract

Yawning is an important prehistorical physiological mechanism that exist in animals across a variety of species. In the past 50 years, investigation of spontaneous yawning and contagious yawning has been ongoing. Several hypotheses were introduced to this field, such as the circadian hypothesis, brain-cooling hypothesis, and oxygenation hypothesis. These theories now trigger growing debates, and each theory are supported by their own evidence. Here, this review examines three popular hypotheses and reveals the arguments among them. Conversely, one novel hypothesis of mental stress seems more convincing than the other three hypotheses, and needs to be examined by empirical studies.

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“…Numerous functions have been proposed for yawning, such as stimulating and facilitating arousal during state changes [ 30 ], increasing mental efficiency [ 31 , 32 ], releasing tension [ 33 ] and nonconsciously communicating psychological stress [ 34 ] or drowsiness [ 35 , 36 ]. While it is true that yawns occur in the transitions between rest and wakefulness [ 35 ], recent research has hypothesized that yawning could be triggered by mental and physical stress [ 37 – 39 ]. This proposal resonates well with human studies observing increased yawning rates before anxiety-provoking and stressful situations [ 29 ] and increased cortisol levels after yawning, although the study was tested in small sample size [ 33 ].…”