You are thinking about consciousness as it's contents. Drugs or brain damage change its contents, but don't change the presence of consciousness. Therefore that only proves that the contents are physical.

If you consider sleeping or fainting as loss of consciouness, I wouldn't be sure that's the case. Perhaps what happens then is that we lose the perception of the objects of consciousness and we are conscious of a blank state, and as we have no point of reference and nothing to know we mistakenly think that we were 'unconscious', while we were conscious of nothing.