Why is it again that we yawn?

This question is probably as old as yawning itself and still remains to be answered, although there are a lot of interesting hypotheses out there:

1. Respiratory and circulatory hypothesis
The most widely believed and oldest hypothesis that yawning functions to regulate levels of oxygen and carbon dioxide. However, studies by Provine and colleagues showed that when measured in a controlled environment, yawning frequency was not affected by manipulating levels of oxygen and carbon dioxide [1].

2. Mental attribution hypothesis and social/communication hypothesis
This hypothesis claims that just thinking about yawning makes you yawn [2] and is closely linked to the social/communication hypothesis. Yawning is understood across cultures and frequently occurs in social contexts. Moreover, everyone has experienced a yawn being "contagious". Interestingly, the susceptibility to contagious yawning correlates with empathic skills. This is reduced, for example, in patients with autism or schizophrenia. However, experimental data are lacking and highly debated [3,4].

3. Arousal hypothesis
This hypothesis proposes that yawning facilitates arousal. The experimental data suggest that yawning indeed occurs during progressive drowsiness or sleepiness, which would be compatible with the hypothesis. However, EEG data could not show specific arousing effects of yawning on the brain or the autonomic nervous system [4].

Macaca fuscata juvenile yawning via Wikimedia Commons

4. Thermoregulation hypothesis
This is one of the most recent hypotheses and puts forward that yawning functions as a brain cooling mechanism. The mechanism proposed being an increase in brain temperature triggers a yawn and the physiological reactions following promote a return to brain thermal homeostasis. However, again, experimental data remain sparse and highly debated [3,4].
Nevertheless, here are a few facts that we know for sure: Yawning is a phylogenically old property and occurs in four vertebrates classes: mammals, reptiles, birds and fish. A yawn is characterized by a standard cascade of movements over a 5- to 10-second period. The mouth opens widely for 4 to 6 seconds with simultaneous retraction of the tongue, and is usually combined with retroflexion of the head and sometimes elevation of the arms. Numerous neurotransmitters, neuropeptides, and hormones have been found to be implicated in the control of yawning; amongst others, dopamine, acetylcholine, glutamate, serotonin, nitric oxide, adrenocorticotropic hormone related peptides, oxytocin, and steroid hormones facilitate yawning, whereas opioid peptides have an inhibitory effect [4].

What is there to be concluded? 
Ladies and gentlemen, there's research to be done!
Do you also sometimes wonder about the simple neuroscientific questions in everyday life, but don't really feel like looking them up right away?

Do you also sometimes wonder about the simple neuroscientific questions in everyday life, but don't really feel like looking them up right away? For questions like this, just mail us your question (cns-newsletter@charite.de) and Dr. Harebrained will give us his explanation!


[1] Provine et al., Behav Neural Biol, 1987
[2] Provine, Ethology, 1986
[3] Gallup, Neurosci Biobehav Rev, 2011
[4] Guggisberg et al., Neurosci Biobehav Rev, 2010


By Veronika Lang, originally published June 2012 – Volume 05, Issue 2, "Open Access"