The sentence is : They rest sixteen to eighteen hours a day and spend most of that unconscious.
It comes from a passage about koalas and why they are so lethargic. Well, the sentence is complete without 'unconscious,' so it seems to have to be followed by an adverb not an adjective. But, if the adverb, unconsciously, is used, I think, it specifies the manner in which the verb phrase is done; the way of their spending time is unconscious. But here, what is meant by 'unconscious' was the state of the subject, they, during their spending time. And thus, I think the adjective, unconscious, is the right choice. But I'm not a native speaker, so I just want to make sure if the alternative sounds really bad, which would be "They rest sixteen to eighteen hours a day and spend most of that unconsciously."
Also, at the same time, I'm in a position to explain about this sentence, but I can't think of grammatical phenomenon for this. The closest one that I can think of is what is called quasi-complements, like the adjectives at the end of the following sentences. He died young. Or, they arrived exhausted. In these examples, the adjectives young and exhausted specify the subjects after intransitive verbs. But I heard that these quasi-complements occur after these types of intransitive verbs, such as die, arrive, etc. Unfortunately, that is not the case with the sentence that I'm struggling with. Could you explain to me what grammatical rule made this sentence possible? Thank you so much for your help!