Ok. I'm not a doctor or anything even similar, so take my thoughts/questions for what they're worth.
Could dizziness, sleepiness and nausea felt after a massage simply be something akin to "overstimulation"? Stimulating blood flow to all other parts of the body removes some of the blood flow to the brain? Sort of similar to the mental state you get into after a big meal.
What about the mental stimulation of it all? I know it feels relaxing to us, but how does the brain interpret all that sensation? I mean, you've got (probably) a complete stranger doing strange things to your body, some of them kind of hurt, sometimes, and you're laying there partly undressed and vulnerable. Could our brain be getting confused a bit as to whether we're being attacked, sexually stimulated, or some other kind of sensation as yet unexperienced that causes us to experience symptoms similar to a mild case of shock?
I'm curious as to whether people who have had many massages and get them often experience the same kind of effects, or if they get used to it, so it doesn't affect them that way after a while.
Which then they chalk it up to "having good Qi"..
Meg