i did a search for this and couldn't find anything. why do i always yawn when i first take speed or e? just curious. i know why you're body usually yawns...to get oxygen to your organs. but why do i yawn when i do any amphetamines?
However, if our bodies make us yawn to drawn in needed oxygen, wouldn't we yawn during exercise? Robert Provine, a psychologist at the University of Maryland, Baltimore County, and a leading expert on yawning, has tested this theory. Giving people additional oxygen didn't decrease yawning and decreasing the amount of carbon dioxide in a subject's environment also didn't prevent yawning.
It seems that a whole bunch of neurotransmitters/peptides influence yawning with opiates surpressing it and serotonin, dopamine amoung others stimulating it.
yawning is under the control of several neurotransmitters and neuropeptides at the central level as this short overview of the literature on the neurochemistry of yawning shows. Among these substances, the best known are dopamine, excitatory amino acids, acetylcholine, serotonin, nitric oxide, adrenocorticotropic hormone-related peptides and oxytocin, that facilitate yawning and opioid peptides that inhibit this behavioral response.
from here.