Its the Abudraham.
Tumah is split up into two sources. A) Zohar- Sleep brings tumah.
B) Gemorah in Shabbos implies that the night time brings tumah, even without sleep.
One more thing- the rashba is brought down by the MB as one of two main reasons. That and the Rosh-cleanliness. Not tumah as you say.
Just to clarify, the idea of having to wash hands before walking 4 amos is only tied to the fact that the tumah is very strong and we dont want it lingering on our hands.
Kedusha pushes away tumah- a fairly simple concept.
The Gr"a said that the amount of kedusha that the graf "created" with his kidush hashem, pushed away some of the tumah of yodayim, this is why ppl have a tradition of not being machmir on the
halacha of 4 amos.