I have this too
I see it as explained only by one theory
that exess adrenaline we secrete causes it - as well as photophobia, sensitivity to smells nervousness, feeling " tired but wired" at night and maybe also our tight muscles
adrenaline is the flight or fight hormone so dilates pupils, heightens the senses for danger tenses the muscles, decreases peripheral blood supply and many other things
if you have POTS/OI you secrete extra adrenaline to help you stay upright ; your body is then stuck witha ll this extra adrenalin and its many side effects - all day - not good
sorry reply was deleted 4 times now so cutting it short - see this link for the theory though and good luck
http://forum.notcrazy.net/index.php?topic=9571.0