Just a little something I found while digging up information on I/E. Much more random information to come in the future.
According to the study by David Lester and Diane Berry (Autonomic nervous system balance and introversion. Perceptual and Motor Skills Dec. 1998 Part 1, Vol. 87 Issue 3, pg. 882), introverts have more balance between their parasympathetic-sympathetic nervous system as opposed to extraverts, where such a balance is indicated by "salivat[ing] a lot versus mouth dry, blood pressure below versus above average, taking things easy versus being on the go, pupils smaller versus larger than average, feel[ing] hungry quite often versus not very often, urinat[ing] frequently versus go[ing] long periods without doing so, and hav[ing] a fairly dry skin versus sweat[ing] alot". Statistically, autonomic nervous system balance has between a moderate and low negative correlation with Extraversion (r = -.30), but one that is enough to show a general predominance of this balance in introverts.
Anyway, I think the results correlate pretty well with our standard definition of Introversion, that is being more sensitive to the dynamics of one's inner, subjective state. An introvert would keep better track of himself, thus consistently striving for a balance in the autonomic nervous system's state. Any comments?