[Avodah] What is the basis of the requirement to wash ones hands upon awakening

[Prof. Levine]

At 02:25 PM 9/14/2018, Micha Berger wrote:
>3. When one sleeps, a ru'ach ra'ah descends upon the body, and when he
>: awakens, the ru'ach ra'ah remains on his hands, and is removed by the
>: ritual of washing the hands three times (Bais Yosef O.C. 4 from Zohar
>: and Mechaber 4:2 with Mishnah Berurah s.k. 8).
>
>Al pi nistar, however, one has to wash as soon as one can, and as
>metaphysics is involved we care about how you wash beyond just whether
>it's koach gavra.
>
>It is not an issue of taharah as prep for a mitzvah as much as tum'ah
>is a state to avoid being in.

Why does the ru'ach ra'ah remain only on the hands?  How does it 
leave the rest of the body?

YL
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