(Here is what I have found on yawing. So your doc. is right. There is no real information to know the cause. But these are the speculations that I have found. Hope this helps)
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 deep inhalation and 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, yawing has the infectious quality, i.e. seeing a person yawing, or just thinking yawing, can trigger yawing which is typical example for positive feedback. 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 were noted yawning right before their first jump.
Hypothesized causes of yawning
The deep inhalation during a yawn is means of preventing alveolar collapse within the lung.
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.
A means of cooling the brain.
An action used as an unconscious communication of psychological decompression after a state of high alert.
An excess of carbon dioxide and lack of oxygen in the blood. 
A way of displaying (or indicative of) apathy.
A means of equalizing middle ear pressure, which can be triggered by another's yawning.
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 behaviour 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 behavioural 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 behaviour 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 will cause another person to "sympathetically" yawn. Observing another person's yawning face (especially his/her eyes), or even reading about or thinking about yawning, can cause a 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 of. 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 yawning cat
 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.[citatio
Severe Central & Obstructive Sleep disordered breathing
Lowest Sat 85% High96.1%
Total Central,Obst,& mixed 190/hr
Out of that total 66 were Hypopneas
IpapMin7 max16 Epap6
Bipap Auto SV
Resmed Mirage Swift II