Tanbark was originally any kind of bark (usually an oak species) which had been soaked to produce a tanning solution. It was waste so it was fairly cheap. Later it became a generic name for shredded bark.
IIRC tan oak took its name from the fact that the Californians used it extensively to tan hides because it was so common. Under Mexico hides, tanned or untanned were a major item of commerce in old California.
I don't believe tan oak is even a true oak. But it's bark is very high in tannin.
--RC Sleep? Isn't that a totally inadequate substitute for caffine?