Because a man rising in the morning from his bed is like a new creature in as far as the service of the Creator is concerned, Baruch Hu, it is incumbent upon him to purify himself and wash his hands out of a vessel, just as the Kohen was accustomed to purify his hands daily out of the wash-basin prior to his service (in the Temple). This washing may be supported by a Biblical verse; for it is said:
"I will wash in innocence my hands, and I will compass Your altar O Hashem; That I may publish with a loud voice" etc. (Tehillim 26:6-7).
There is another reason for this morning washing, when a man is asleep the holy soul departs from his body, and the unclean spirit comes down upon him. When he rises from his sleep the evil spirit departs from his entire body excepting from his fingers. From there the unclean spirit does not depart unless he spills water on them three times alternately. One is not permitted to walk four cubits without having his hands washed, except in extreme cases of necessity. (Kitzur Shulchan Aruch, Laws Relating to the Conduct Upon Rising in the Morning)
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Baruch Hu - Blessed is He
Kohen - Priest
Tehillim - Psalms
