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When phylogenetically primitive structures are disinhibited, they regain autonomy in the homeostatic process associating the massive inspiration of yawning--a form of behaviour that stimulates vigilance--with a motor control mechanism that is active during locomotion. For this phenomenon, we coined the term 'parakinesia brachialis oscitans'.

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Yawning is a behavior to which little research has been devoted. However, its purpose has not yet been demonstrated and remains controversial. In this article, we propose a new theory involving the brain network that is functional during the resting state, that is, the default mode network. When this network is active, yawning manifests a process of switching to the attentional system through its capacity to increase circulation of cerebrospinal fluid (CSF), thereby increasing clearance of somnogenic factors (prostaglandin D(2), adenosine, and others) accumulating in the cerebrospinal fluid.

Yawning is a physiological behavior, an emotional stereotypy that indicates the homeostatic process of the mechanisms regulating rhythms, such as sleeping/waking, hunger/satiety or mating/relaxation, generated by the diencephalon. As with all physiological behaviors, its deregulation reveals disorders. In daily practice, yawning as a symptom is generally neglected. That is why we propose a wide overview of yawning in diseases, its consequences and significance.

Yawning is a physiological behavior and, as with all such behaviors, its deregulation is indicative of an underlying disorder. This chapter breaks this topic down into the types of yawning (incomplete, absent or excessive) and the direction of causality (triggering or relieving a disorder).

The capacity of four-dimensional sonography to evaluate complex facial expressions allows recognition of a common behavior, yawning. Although there has been remarkably little interest in yawning in research and medical practice, even though it is an everyday phenomenon, we submit an original interpretation on the basis of knowledge derived from phylogeny and ontogeny. As a flip-flop switch, the reciprocal interactions between sleep-and wake-promoting brain regions allow the emergence of distinct states of arousal. By its ontogenical links with REM sleep, yawning appears as a behavior which procures an arousal reinforcement through the powerful stretch and the neuromuscular rewiring induced. Yawning indicates a harmonious progress in the development of both the brainstem and the peripheral neuromuscular function, testifying to the induction of an ultradian rhythm of vigilance. The lack of fetal yawn, frequently associated with lack of swallowing, associated or not with retrognathia, may be a key to predict a brainstem's dysfunction after birth.