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A yawn is a reflex of deep inhalation and exhalation associated with being tired, with a need to sleep, or from boredom. The word "yawn" has evolved from the Middle English word yanen, an alteration of yonen, or yenen, which in turn comes from the Old English geonian. Yawning is a powerful non-verbal message with several possible meanings, depending on the circumstances:

1. It can be an indication of tiredness, stress, over-work or boredom.
2. An action indicating psychological decompression after a state of high alert.
3. A means of expressing powerful emotions like anger and rejection.

A yawn can express strong anti-social messages, and so in some cultures people try to mute or mask them by placing a concealing hand over the yawning mouth.

1 Causes of yawning

A long-standing theory behind yawning is that yawning is caused by an excess of carbon dioxide and lack of oxygen in the blood. The brain stem detects this and triggers the yawn reflex. The mouth stretches wide and the lungs inhale deeply, bringing oxygen into the lungs and thence to the bloodstream. It is almost certain however, that this hypothesis is not correct; a more recent theory is that yawning is used for regulation of body temperatureTemperature is the physical property of a system which underlies the common notions of "hot" and "cold"; the material with the higher temperature is said to be hotter. General description The formal properties of temperature are studied in thermodynamics.. Another theory is that yawns are caused by the same chemicals (neurotransmitters) in the brainFor other articles about other subjects named brain see brain (disambiguation). In the anatomy of animals, the brain or encephalon is the supervisory center of the nervous system. Although the brain is usually cited as the supervisory center of vertebrate that affect emotions, moodA person's mood is a sustained affective state, consisting of a combination of emotions. In normal functioning, moods are largely adaptive to external events. A mood disorder (such as clinical depression) is a pronounced maladaptive mood. Similarly, seaso, appetiteThe appetite is the desire to eat food, felt as hunger. It exists in all higher lifeforms, and serves to regulate adequate energy intake to maintain metabolic needs. It is regulated by a close interplay between the digestive tract, adipose tissue and the and other phenomena. These chemicals include serotoninSerotonin Chemical name5-Hydroxy-tryptamine or3-(2-aminoethyl)-1''H indol-5-ol Chemical formulaCHNO Molecular mass176. 22 g/mol CAS number50-67-9 SMILESNCCC1 CNC2 C1C C(O)C C2 Serotonin 5-hydroxytryptamine or 5-HT is a monoamine neurotransmitter synthesis, dopamineDopamine is a neurotransmitter in the brain. As a member of the catecholamine family, dopamine is a precursor to epinephrine ( adrenaline) and norepinephrine ( noradrenaline) in the biosynthetic pathways for these neurotransmitters. Dopamine is synthesize, glutamic acidGlutamic acid or glutamate is one of the 20 most common natural amino acids. As its name indicates, it is acidic, with a carboxylic acid component to its side chain. Glutamic acid is critical for proper cell function, but it is not considered an essential and nitric oxide. As more 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.

The yawn reflex is often described as infectious: if one person yawns, this will cause another person to "sympathetically" yawn. The reasons for this are unclear, possibly due to the "power of suggestion". Other theories suggest that the yawn serves to synchronize mood behavior among gregarious animals, similar to the howling of the wolf pack during a full moon. It signals tiredness to other members of the group in order to synchronize sleeping patterns and periods of activity. It can serve as a warning in displaying large, canine teeth thus proclaiming, "don't attack while I am sleeping."