Hi! "you should have fallen asleep, shouldn't you?" is the correct sentence because the tag question "shouldn't you?" repeats the modal verb "you should" in the first part of the sentence. "you should have fallen asleep. isn't it?. " is incorrect because you can't use the tag question with the verb to be "isn't it?" with a sentence that has a modal verb (in this case "should"). If you wish to use "isn't it?" as the end question then the main verb must be the verb to be, for example. "It *is* cold today, isn't it?" or "That *is* correct, isn't it?". Hope this helps! :)

Hi Kai, The first one is correct!

you should have fallen asleep, shouldn't you've? the end question should match the beginning of the sentence.