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dmmhttp://www.scholarpedia.org/article/Yawn
"The yawn is a stereotyped and often repetitive motor act characterized by gaping of the mouth accompanied by a long inspiration of breath, a brief acme, and then a short expiration of breath."
"Stretching and yawning (known as pandiculation when they occur together) are under-researched features of behavior. Ethologists agree that almost all vertebrates yawn (Deputte, 1974). Yawning is morphologically similar in reptiles, birds, mammals and fish. These behaviors may be ancestral vestiges maintained throughout evolution with little variation (phylogenetic old origins)..."
Neurophysiology
"A good number of clinical and pharmacological arguments indicate that yawning involves the hypothalamus (particularly the paraventricular nucleus, PVN), the bulbous and pontic regions of the brainstem, with frontal region connections in primates and to the cervical medulla (Argiolas & Melis, 1998). The PVN is an integration centre between the central and peripheral autonomic nervous systems. It is involved in a number of functions ranging from feeding, metabolic balance, blood pressure and heart rate, to sexual behavior and yawning. In particular, a group of oxytocinergic neurons originating in this nucleus and projecting to extra-hypothalamic brain areas (e.g., hippocampus, medulla oblongata and spinal cord) controls both yawning and penile erection (Kita et al., 2006).
Activation of these neurons by dopamine and its agonists, excitatory amino acids (N-methyl-D-aspartic acid), oxytocin itself, or by electrical stimulation leads to yawning..."***** Makes one sleepy, does not it?
