Yawning and airway physiology: a scoping review and novel hypothesis

Yawning, gaping, or oscitation is a common and stereotyped physical behavior performed by most vertebrates, including mammals, birds, reptiles, amphibians, and even fish [1, 2]. In most animals, a regular yawn lasts 4 to 7 s, and is characterized by (1) a long inspiratory phase with gradual mouth gaping, followed by (2) a brief climax (or acme) with powerful muscle stretching, and (3) a rapid expiratory phase with muscle tension release [1, 3, 4]. For fish and birds, this is described as gradual mouth gaping, staying open for at least 3 s and subsequently a rapid closure of the mouth [5]. A yawn is performed from fetal stages to old age, in human fetuses starting from the 11th or 12th post-conceptional week [6, 7].

Fetuses yawn around 25 times per day, [8] after which the frequency decreases with age [9]. Adults have been observed to yawn about nine times per day (range 0–28) [10], often in fits of two or three with increasing intensity [3, 11]. Yawning frequencies of animals have hardly been studied, but convey the impression of being similar [12, 13]. Yawning’s frequency is influenced by multiple internal and external triggers. Primarily, a yawn is triggered in a low-vigilance state of the brain while transiting between wake and sleep. Both the awaking process and progressive drowsiness regularly provoke a yawn [1, 9, 14]. Particular stressful events (e.g., athletes before a game), imitation of a yawn, and hungriness have also been reported as endogenous triggers [3, 10]. In humans and certain social animals, yawning is sometimes triggered by exogenous triggers: seeing or hearing another individual yawn, or reading about yawning, which you may have just experienced [5, 15, 16]. This is defined as contagious yawning and can be suppressed with difficulty. Opioid withdrawal syndrome, psychoactive drugs (e.g., apomorphine, naloxone after morphine), and neurological diseases (e.g., amyotrophic lateral sclerosis, multiple sclerosis) are related to excessive yawning, which is defined as more than 3 yawns per 15 min [17, 18]. Opioid peptides are known to inhibit a yawn, [19] and some psychotic disorders may be related to decreased yawning, of which no cutoff value has so far been reported [20].

Yawning is a complex neurological act, mainly orchestrated in the brainstem near the basic life centers for breathing, swallowing, mastication, and coughing [19]. Top-down control by cortical, limbic, and hypothalamic centers is involved in triggering yawning and voluntary inhibition in certain intelligent animals [19]. A variety of neuroactive agents have been identified to contribute to a yawn: nitric oxide, dopamine, acetylcholine, glutamate, serotine, adrenocorticotropic hormone (ACTH), oxytocin, and steroid hormones [19]. Based on lesion studies and neuropharmacological studies, effector neurons are thought to be the cranial nerves V, VII, IX, X, XI, and XII (innervating the masseter, facial, pharyngeal, laryngeal, neck, and tongue muscles respectively), the cervical nerves (innervating the diaphragm and scalene muscles), and thoracic nerves (innervating the intercostal muscles) [19, 21]. These neurons fire in a unique rhythmical order, resulting in a complex pattern of contracting muscles around the respiratory and most proximal digestive tract (anatomical overview in Fig. 1).

At first, gradual inspiration is caused by contraction of the diaphragm and intercostal muscles. Then, the mandible lowers, and the mouth may open, defined as “gaping” (one can yawn without opening the mouth; however, this is less satisfactory). The tip of the tongue retracts and moves downward, accompanied by prominent downward movement of the larynx and hyoid bone, which is achieved by contracting infrahyoidal muscles (Fig. 3) [3, 22]. Subsequently, the inspiration accelerates and the soft palate and posto-lateral side of the pharynx elevate, which is the point where a turbulent inhaling sound of the palate and clicking sounds of the opening eustachian tubes may be experienced [23]. At this point, the pharynx diameter has increased 3–4 times [1, 3, 24] and the hyoid bone has reached its nearest position to the mandible. Peak forces of dilator muscles (agonists) initiate the acme phase, where jaw and pharyngeal muscles (antagonists) are maximally stretched, often accompanied by stretching of other body muscles (e.g., trunk, arms) [3, 25]. This is also known as the stretch-yawning syndrome, or pandiculation (in Latin pandare = stretching), which is primarily observed when awakening [3]. This powerful stretching and maximal dilation of the airway from the mouth to alveolus takes 1 s or less and comprises the acme phase [26]. A facial grimace, closed eyes, and sometimes lacrimation might be observed. After the climax, muscle tension is released and expiration follows. This may be experienced with vocalization and a rewarding sensation [27]. The morphology of a yawn may, however, depend on the (social) context [28].

The function of yawning’s complex motor pattern is widely debated. Four popular theories describe the function of yawning and indicate there is no consensus about yawning’s main function. First, the “brain arousal hypothesis” suggests yawning diffusely activates the brain [1]. However, brain arousal by yawning is not evident in EEG, skin conductance, and other autonomic parameters [29]. If present in studies, arousal might well be triggered by voluntary body movements [30]. Second, the “respiration hypothesis” describes yawning as an oxygenating maneuver, primarily to supply the brain [31]. This theory has been discarded since Provine et al. (1987) could not trigger yawning in students by exercise and by breathing through a hand-held mask with increased CO₂ levels (5%). Likewise, stimulated yawns could not be inhibited by high (100%) O₂ levels. This study mentioned no O₂ or CO₂ pressures in participants, but significant changes were not expected by Krestel et al. (2018). Therefore, a possible oxygenation function would not explain (frequent) yawning in utero. This oxygenating theory underlies the theory thata yawning supports short-term oxygenation (or saturation), while we suggest yawning enables ventilation by widening the airway, which may be interpreted as long-term oxygenation (see “Discussion”). Third, the “communication hypothesis” is based on the contagiousness of yawning and advocates for a communicating or group synchronization function [27]. Social animals only yawn contagiously for around 10% of episodes [14], while many other animals do not yawn by external stimuli at all [19]. Social interaction as a main function of yawning is not therefore supportable, though social interaction may well co-exist as a minor function. Fourth, the “brain cooling hypothesis” suggests the brain temperature decreases with yawning [32]. A recent publication of Massen et al. (2021) supported this hypothesis by relating the frequency of yawning with brain mass [2]. However, a significant decrease in temperature during a yawn is physically impossible according to critical review and calculations by physiologist Elo [33].

Yawning is one of the most rarely studied behaviors resulting in heterogenous studies with low level of evidence. Determinants and outcomes of included studies were variable, which makes identifying relationships between studies fairly complex. Exclusion criteria were arbitrary, as “increased” and “decreased” yawning has not yet been defined and was therefore often based on the subjective view of authors. Symptoms of obstructive airway conditions varied widely and may have different relationships with yawning, but did not have objective parameters. Distinct yawn patterns and intensities were not documented. These parameters  however may also influence obstructive airway conditions, instead of only frequency of yawning. Dilator forces should be within the scope of future research, as they are never or seldom described. The temporal relationships in Table 1 should not be interpreted as causal factors yet, as multiple confounding factors are present and influence both the yawning frequency and the airway condition (e.g., low-vigilance state, thinking of yawning, and medication). Therefore, these relationships cannot be extrapolated to yawning’s function. Further research is necessary to investigate the following causal relationships: (A) decreased yawning leads to more obstructive airway, (B) increased yawning leads to less obstructive airway, (C) more obstructive airway leads to increased yawning, (D) less obstructive airway leads to decreased yawning.

We combine these modest results with yawning’s characteristics (see “Introduction”) to come to a novel hypothesis on the function of yawning.

The airway physiology hypothesis entails:

Yawning secures long-term oxygenation by creating an enlarged airway lumen by muscle repositioning, primarily in the upper airway. A single letter-to-the-editor by Hanning, [52] which was found during this literature search, mentioned this same hypothesis; however, the mention was brief and without discussion.

The airway is a hollow and dynamic passage, which must remain open at all costs. The airway lumen is continuously subject to forces of gravity, changing respiratory pressures, and body movements. Therefore, the airway is largely supported by cartilage and bony structures (black in Fig. 2). However, similar fixation of the airway would not be functional in the pharynx and lungs (blue in Fig. 2). The dynamic movements of the pharynx and lungs are essential, and distinctively mentioned below.

Pharyngeal muscles determine the diameter and volume of the upper airway. These muscles interact in a complex fashion to constrict and dilate for different purposes: securing the airway, swallowing, and vocalization. Upper airway patency (or volume) is regulated by position of the tongue, the hyoid apparatus, the posterolateral pharyngeal walls, and the soft palate [53]. Muscles responsible for these positionings are illustrated in Fig. 3 in yawning condition. Involved muscles must be perfectly balanced, as both loose and tight pharyngeal muscles result in a disturbed balance, and could lead to higher airflow resistance, higher respiratory effort, and potential apnea [54]. Besides endangering respiration, proper swallowing and vocalization may also be at risk due to muscle imbalance. Yawning (now considered repetitive airway stretching and dilation) may be the ideal maneuver to restore this muscle balance. The entire airway dilates maximally during a yawn, which automatically results in powerful stretching of the jaw and swallowing muscles. This is illustrated in Fig. 3: in blue stretching constrictive pharyngeal muscles, and in red tightening dilator muscles. In basic muscle physiology stretching is known to help elongate muscles (prevent tightening), create comfortable and functional muscle tone, and increase range of motion [55]. This is well achieved by gradual eccentric contraction in fits of two or three with increasing intensity (see “Introduction”). Post-yawn widening is not well documented. But video-endoscopic findings of Titze [56] and Boone [22] showed that the pharyngeal lumen maintains a widened position after a yawn. This automatically decreases airway resistance and reduces the chance of collapse [53]. Significant widening and stretching during a yawn and an increase of airway lumen size post-yawning substantiate this hypothesis. However, these studies must be repeated with better imaging in the future.

What neural feedback mechanism may be responsible to anticipate on airway collapse? Pharyngeal sensory feedback via the vagal nerve was suggested by Evans (1978), who described two cases of choking children who yawned frequently, and stopped yawning after obstruction was relieved. We suggest that proprioceptive feedback from pharyngeal muscles is essential information to orchestrate a complex muscle balance. Proprioceptive feedback responsible for triggering a yawn is demonstrated by voluntarily opening the jaw repetitively and inhaling gradually (introduction). This imitation alters pharyngeal muscle tones, temporarily narrows the airway, and triggers a yawn in most individuals [3, 19, 22]. Other triggers of yawning discussed in the “Introduction” may also result from reduced muscle tone and accompanying proprioceptive feedback. If extrapolated, the beginning of a yawn might even be the trigger for continuation of a yawn (or the “real yawn”). Relative frequent yawning of children may also be explained by their relative narrow airway, which makes frequent anticipation crucial. Whether or not there is a relationship of airway patency (indirectly implicating yawning) and sudden infant death syndrome (SIDS) may be a scope of future research.

In the lungs, compliance is essential for ventilation by alternate expansion and shrinkage. This compliant property may, however, also lead to bronchiolar and alveolar collapse (atelectasis). This happens during sleep, when shallow breathing and relative hypoventilation are common due to lowered oxygen demand [59]. Deep inspiration during a yawn, in which lung volume increases 300–400% when compared to tidal volume, [26] pops-open the small airways in the lungs and secures long-term oxygenation. This was also suggested by Cahill et al. (1978) and Walusinski et al. (2006). Bartlett et al. [38] stated that yawning is functional in preventing or treating atelectasis by prolonged maximal inflation. Sighing and coughing are accepted maneuvers to prevent and treat atelectasis. However, the yawn maneuver needs further investigation to imply such a purpose. Preventing atelectasis may also be interpreted as a minor function of yawning, comparable to non-verbal communication, mid-ear pressure clearing, and other advantageous “side effects.”

Another important feature of yawning may affect fetuses, who yawn frequently from the first trimester to birth. Provine (2005) proposed that fetal yawning may sculp the jaw joint, while others stated it may help in spreading the surfactant in utero. Some stated it may only serve as preparation for later life [19]. To extend our hypothesis, fetal yawning may help airway development by repetitive dilation and muscle repositioning. The airway must be in perfect condition to anticipate on one of the most critical moments in life: the first breath.

Yawning is executed by almost all, if not all vertebrates, which implies a physical function shared by all members of this group. One similarity is that all members have one hollow structure used for both dilator and constricting purposes: oxygenation by airflow or water current versus digesting by biting, chewing, and peristaltic movements. This is why this hollow structure is dynamic and uniquely modifiable by interacting muscles. These distinct physiological functions demand opposite muscle movements with limited fixation to the surrounding. Oxygenation remains the top priority, which is why strong repositioning movements must take place to restore a vital, dilated balance. If correct, one needs little imagination to realize yawning has likely been executed for millions of years, even by dinosaurs.

If indeed, yawning has an important role in airway physiology, yawning will also have beneficial effects in patients with a collapsible or obstructing airway. For example, yawning may potentially counteract respiratory complications due to opioid use (which inhibits yawning) and OSA, and it may well influence swallowing disorders. To sustain these speculations, much more about yawning needs to be studied. Suggestions for further research include (1) imaging for precise determination of muscles involved before, during, and after a yawn; (2) imaging and/or flow measurements of airway resistance and volume before and after a yawn; (3) RCTs of the effect of stimulated yawns on OSA, swallowing disorders, and postoperative pulmonary complications (e.g., atelectasis); and (4) animal trials of respiratory effects resulting from absent or decreased yawning.