8 Answers

             MAN I'M  TIRED AND SLEEPY !!!!!!

You're tired.Go to bed.Get some sleep.

When you yawn you are cooling your brain down. That is the scientific view.

Lack of sleep, need of more oxygen, boredom, lots of things.

You all are right.  It's very interesting.  I don't think anyone really knows why we yawn.  Below are various explanations.

From www.wikipedia.com:  A yawn is a reflex of simultaneous inhalation of air and stretching of the eardrums, followed by exhalation of breath. Pandiculation is the act of yawning and stretching simultaneously.

Yawning is commonly associated with tiredness, stress, overwork, lack of stimulation and boredom. In humans, yawning is often triggered by others yawning (e.g. seeing a person yawning, talking to someone on the phone who is yawning) and is a typical example of positive feedback. This "infectious" yawning has also been observed in chimpanzees and dogs

Proposed causes

One states that yawning occurs when one's blood contains increased amounts of carbon dioxide and therefore becomes in need of the influx of oxygen (or expulsion of carbon dioxide) that a yawn can provide,[4] but studies have since shown it to be either incorrect or, at the very best, flawed.[6] Yawning may in fact reduce oxygen intake compared to normal respiration, not increase it.[7]

Another speculated reason for yawning is the desire to stretch one's muscles. Yawns are often accompanied by the urge to stretch. Prey animals must be ready to physically exert themselves at any given moment. There have been studies that suggest yawning, especially psychological "contagious" yawning, may have developed as a way of keeping a group of animals alert.[9] If an animal is drowsy or bored, it may not be as alert as it should to be prepared to spring into action. Therefore, the "contagious" yawn could be an instinctual reaction to a signal from one member of the "herd" reminding the others to stay alert. Nervousness has also been suggested as a possible reason. Nervousness often indicates the perception of an impending need for action. Anecdotal evidence suggests that yawning helps increase the state of alertness of a person. Paratroopers have been noted to yawn in the moments before they exit the aircraft.

Research data strongly suggest that neither contagious nor story-induced yawning are reliable in children below the age of six years.

Another notion states that yawning is the body's way of controlling brain temperature. In 2007, researchers, including a professor of psychology, 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. Similarly, a study by Jared Guttmann at Worcester Polytechnic Institute found that when a subject wearing earplugs yawned, a breeze is heard caused by the flux of the air moving between the subject's ear and the environment. Researcher Guttmann determined that a yawn causes one of three possible situations to occur: the brain cools down due to an influx or outflux of oxygen, the pressure in the brain is reduced by an outflux of oxygen, or the pressure of the brain is increased by an influx of air caused by increased cranial space.

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 opioid neurotransmitters such as endorphins reduces the frequency of yawning. Individuals in opioid withdrawal exhibit a greatly increased 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" (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 Garrett Norris, 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 Garrett.

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

It is also possible that yawning is a less potent territorial reflex. Usually being associated with boredom or lack of interest, yawning is often displayed when the subject is faced with a worrying or dangerous situation. Therefore, to yawn in the presence of a rival for territory would portray the subject as unthreatened by the rival in order to deter it from entering the subject's territory. This behavior is seen in many primates as well as some feline species.[citation needed]

To look at the issue in terms of a possible evolutionary advantage, yawning might be a herd instinct. For example, theories suggest that the yawn serves to synchronize mood in 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.

Would you like to know how many times I yawned while reading this thread? About 10. I had to stop reading.  Seriously. Still fighting the urge to yawn. 

There are some innocent and serious medical reasons for yawning.  You can find them by googling "reason for yawning" and check it out.  Sleep disorders, Muscle stretching, Aortic dissections (I quit there...scared me :D)

Just as Yv, Darci, and Ben said, lack of oxygen.