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In this article, it is considered that the mechanism of yawning function is cor-relative to sleep. It was revealed from the electrophysiological studies that the slow wave sleep (SWS) was characterized as decrease in excitation from inspir-atory motoneurons more than from expiration, manifesting the expiratory activity dominating in SWS. To analogize yawning to SWS, herein it is only considered the expiration during yawning, pressing the sensory posterior palate and maxillary sinuses with air flow, so that yawns similarly regulate the vascular circulations from mouth to eye as SWS, as evidenced by the closure of eyelid in sleep and the association of ophthalmic diseases with obstructive sleep apnea. Besides, it is pointed out that the yawning expiration of high muscular tone elicits the muscular constriction at the back of neck, potentiating the expiratory press in mouth stronger than that of SWS. Accordingly, it is hypothesized that the yawning resembles the SWS to play the neurovascular regulation in head driven by expiration. It is speculated that the initiative mouth opening in yawns may have functions other than that considered here. It is expected that this new and neglected function of yawning with expiratory neurovascular regulation in head may add more to understand the complex functions of yawns.

One Neglected Yawning Function like Slow Wave Sleep: The Hypothetic Neurovascular Regulation from Mouth to Eye Driven by Expiration