We shouldn't be using the accusative form of nouns this way?
Of course not! The accusative form is for other cases, not for the verb to be. Please be more cautelous.
from what i know, 'bisy' is for inanimate objects... so how can it sleep?
You got it backwards, it's "kesy" for inanimate and "bisy" for animate. But kesy can be used for animals too, depends on whether or not each speaker considers them "worthy of it".