A yawn (from the Middle English yanen, an alteration of yonen or yenen, which in turn comes from the Old English geonian), is a reflex of simultaneous inhalation of air and stretching of the eardrums, followed by exhalation of breath. Pandiculation is the term for the act of stretching and yawning simultaneously.

Yawning is associated with tiredness, stress, overwork, lack of stimulation, or boredom. Yawning can also be a powerful non-verbal message with several possible meanings, depending on the circumstances. In humans, yawning has an infectious quality, i.e. seeing a person yawning, or just thinking of yawning, can trigger yawning which is a typical example of positive feedback.. Infectious yawning has also been noted in chimpanzees. The exact causes of yawning are still undetermined. The claim that yawning is caused by lack of oxygen has not been substantiated scientifically. Some claim that yawning is not caused by lack of oxygen, for the reason that yawning allegedly reduces oxygen intake compared to normal respiration. Another speculated reason for yawning is nervousness and is also claimed to help increase the state of alertness of a person - paratroopers have been noted to yawn in the moments before they exit the aircraft.

Hypothesized causes of yawning

  1. The deep inhalation during a yawn is a means of preventing alveolar collapse within the lung.
  2. The deep inhalation while yawning stretches type II alveolar pneumocytes, which release the surfactant dipalmitoylphosphatidylcholine (DPPC) into the layer of fluid on the alveolar surface.
  3. A means of cooling the brain.
  4. An action used as an unconscious communication of psychological decompression after a state of high alert.
  5. An excess of carbon dioxide and lack of oxygen in the blood.
  6. A way of displaying (or indicative of) apathy.
  7. Tiredness.
  8. A means of equalizing middle ear pressure, which can be triggered by another's yawning.
  9. Need of food or hunger/ appetite due to reduced level of glucose supplied to the brain.
  10. A state of boredom.

In 2007, researchers from the University of Albany proposed that yawning may be a means to keep the brain cool. Mammalian brains operate best within a narrow temperature range. In two experiments, they demonstrated that both subjects with cold packs attached to their foreheads and subjects asked to breathe strictly nasally exhibited reduced contagious yawning when watching videos of people yawning. A similar recent hypothesis is that yawning is used for regulation of body temperature.

Another hypothesis is that yawns are caused by the same chemicals (neurotransmitters) in the brain that affect emotions, mood, appetite, and other phenomena. These chemicals include serotonin, dopamine, glutamic acid, and nitric oxide. As more (or less) of these compounds are activated in the brain, the frequency of yawning increases. Conversely, a greater presence in the brain of opiate neurotransmitters such as endorphins reduces the frequency of yawning. Patients taking the selective serotonin reuptake inhibitors Paxil (paroxetine HCl) or Celexa (citalopram) have been observed yawning more often. Excessive yawning is more common during the first three months of taking the SSRI's. Anecdotal reports by users of psilocybin mushrooms often describe a marked stimulation of yawning while intoxicated, often associated with excess lacrimation and nasal mucosal stimulation, especially while "peaking" (i.e., undergoing the most intense portion of the psilocybin experience). While opioids have been demonstrated to reduce this yawning and lacrimation provoked by psilocybin, it is not clear that the same pathways that induce yawning as a symptom of opioid abstinence in habituated users are the mode of action in yawning in mushroom users. While even opioid-dependent users of psilocybin on stable opioid therapy often report yawning and excess lacrimation while undergoing this entheogenic mushroom experience, there are no reports in the literature of habituated users experiencing other typical opioid withdrawal symptoms such as cramping, physical pain, anxiety, gooseflesh, etc. on mushrooms

Recent research carried out by Catriona Morrison, a lecturer in psychology at the University of Leeds, involving monitoring the behavior of students kept waiting in a reception area, indicates a connection (supported by neuro-imaging research) between empathic ability and yawning. "We believe that contagious yawning indicates empathy. It indicates an appreciation of other people's behavioral and physiological state," said Morrison.

Yet another theory is that yawning occurs to stabilize pressure on either side of the ear drums. The deep intake of air can sometimes cause a popping sound that only the yawner can hear; this is the pressure on the middle ear such as inside an airplane and when travelling up and down hills, which cause the eardrums to be bent instead of flat. Some people yawn when storms approach, which is a sure sign that changes in pressure affect them.

Some movements in psychotherapy, such as Re-evaluation Counseling or co-counselling treatments, believe that yawning, along with laughter and crying, are means of "discharging" painful emotion, and therefore can be encouraged in order to promote physical and emotional changes.

Yawning behavior may be altered as a result of medical issues such as diabetes and adrenal conditions.


The yawn reflex is often described as contagious: if one person yawns, this may cause another person to "sympathetically" yawn. Observing another person's yawning face (especially his/her eyes), even reading (including this article), or thinking about yawning, can cause you or another person to yawn. The proximate cause for contagious yawning may lie with mirror neurons, i.e., neurons in the frontal cortex of certain vertebrates, which upon being exposed to a stimulus from conspecific (same species) and occasionally interspecific organisms, activates the same regions in the brain. Mirror neurons have been proposed as a driving force for imitation which lies at the root of much human learning, e.g., language acquisition. Yawning may be an offshoot of the same imitative impulse. A 2007 study found that children with autism spectrum disorder do not increase their yawning frequency after seeing videos of other people yawning, in contrast to typically developing children. This supports the claim that contagious yawning is based on the capacity for empathy.

To look at the issue in terms of evolutionary advantage, if there is one at all, yawning might be a herd instinct. Other theories suggest that the yawn serves to synchronize mood gregarious animals, similar to the howling of the wolf pack. It signals tiredness to other members of the group in order to synchronize sleeping patterns and periods. This phenomenon has been observed among various primates. The threat gesture is a way of maintaining order in the primates' social structure. Specific studies were conducted on chimpanzees and stumptail macaques. A group of these animals was shown a video of other conspecifics yawning; both species yawned as well. This helps to partly confirm a yawn's "contagiousness."

Gordon Gallup, who hypothesizes that yawning may be a means of keeping the brain cool, also hypothesizes that "contagious" yawning may be a survival instinct inherited from our evolutionary past. "During human evolutionary history when we were subject to predation and attacks by other groups, if everybody yawns in response to seeing someone yawn, the whole group becomes much more vigilant, and much better at being able to detect danger."

A recent study by the University of London has suggested that the "contagiousness" of yawns by a human will pass to dogs. The study observed that 21 of 29 dogs yawned when a stranger yawned in front of them, but did not yawn when the stranger only opened his mouth.

Non-human yawning

In non-human animals, yawning can serve as a warning signal. For example, Charles Darwin, in his book The Expression of the Emotions in Man and Animals, mentioned that baboons use yawn to threaten their enemies, possibly by displaying large, canine teeth. Similarly, Siamese Fighting Fish yawn only when they see a conspecific (same species) or their own mirror-image, and their yawn often accompanies aggressive attack. Guinea Pigs also yawn in a display of dominance or anger, displaying their impressive incisor teeth, this is often accompanied by teeth chattering, purring and scent marking. Adelie Penguins employ yawning as part of their courtship ritual. Penguin couples face off and the males engage in what is described as an "ecstatic display," their beaks open wide and their faces pointed skyward. This trait has also been seen among Emperor Penguins. Researchers have been attempting to discover why these two different species share this trait, despite not sharing a habitat. Snakes yawn, both to realign their jaws after a meal, or for respiratory reasons, as their trachea can be seen to expand when they do this..


Certain superstitions surround the act of yawning. The most common of these is the belief that it is necessary to cover one's mouth when one is yawning in order to prevent one's soul from escaping the body. The Ancient Greeks believed that yawning was not a sign of boredom, but that a person's soul was trying to escape from its body, so that it may rest with the gods in the skies. This belief was also shared by the Maya.

Other superstitions include:

  • A yawn is a sign that danger is near.
  • Counting a person's teeth while he/she is yawning robs them of one year of life for every tooth counted.
  • If two persons are seen to yawn one after the other, it is said that the one who yawned last bears no malice towards who yawned first.
  • The one who yawns first shows no malice towards those he or she yawns around.
  • In Ancient Mayan civilization, yawning was thought to indicate subconscious sexual desires.
  • In some Latin American, East Asian and Central African countries yawning is said to be caused by someone else talking about you.
  • A yawn may be a sign that one is afflicted by the evil eye (Greece).
  • Frequent yawning by a woman indicates she is likely to be committing adultery, which results in a lack of sleep.

These superstitions may not only have arisen to prevent people from committing the faux pas of yawning loudly in another's presence—one of Mason Cooley's aphorisms is "A yawn is more disconcerting than a contradiction" — but may also have arisen from concerns over public health. Polydore Vergil (c. 1470–1555), in his De Rerum Inventoribus, writes that it was customary to make the sign of the cross over one's mouth, since "alike deadly plague was sometime in yawning, wherefore men used to fence themselves with the sign of the cross...which custom we retain at this day.


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