You need to respot the character for every different heighted (if that’s a word, lmao?) animation. So for your situation, you’d need a spot for sitting up, then another for sitting while sleeping. You might find it handy to use a second character of the same gender in the previous position, simply to find a happy spot you like on relations to the new animation, then remove the excess character when done.
That of course, is completely up to you. It’s just something I do while figuring out heights with sitting + talking or “sitting” and talking 