I was laying in bed thinking, and had a bit of a mini-revelation about why it is that we yawn, and came up with this:
Yawning is the mechanism by which our bodies equalise the pressure of the the blood in the brain with the pressure of the blood in the rest of the body.
It makes sense to have separate independent control of the cerebral vascular system to prevent any surge overload of the delicate system of blood vessels there. Somewhere in the neck in the main blood supply to the brain there must be some kind of valve or thickening/strengthening point which mediates the blood pressure there and protects the brain from the fluctuations caused by our physical exertions.
When we are getting ready to sleep or are tired or inactive, the heart begins to slow and blood pressure falls. The brain recognises the slowdown, and additionally recognises that optimal alertness/consciousness is no longer required (perhaps steady, higher pressure in the brain facilitates consciousness, and less blood supply leads to sleep. I wrote an earlier entry about this some time ago). It makes sense to equalise the pressures between the two systems, so…. we yawn.
I suspect that the physical action of yawning is allowing the valve/compression point area of the blood vessels in the neck to dilalte and allow the pressures to equalise (which is the kind of humming/tension sound we can hear near our ears when we yawn), with an immediate release in tension.
Perhaps, then, insomnia is the result of a malfunction of the “valve” which is meant to equalise the two systems, leading to unwanted higher pressure in the cerebral vascular system, thus promoting consciousness.
Anyway, I’m back off to bed….