Your example "I was lying on the couch the whole day" presupposes that there is a couch present that your hearer can identify; otherwise, you would have said "I was lying on a couch the whole day". Of course, you know that. But there is an obvious difficulty with this account of the presupposition connected with using "the" rather than "a", and that is you can use "the" in talking to, or writing to, someone who is not on the scene at all and could not possibly know about the presence of a couch there.
So here is what is going on (and this holds for presupposition generally). You can use presuppositions which assume certain things are true about a scene in order to convey to a listener that those things may be assumed to be true. If you want to convey that there is a single couch in your surroundings that you could lie down on, you'll use "the couch". Otherwise, if you don't want to convey that information, you'll use "a couch".
So the difference between using "a couch" and "the couch" in your example does not have anything directly to do with what is actually in your surroundings at the time you're referring to, but rather what you would like your listener or reader to assume about those surroundings.