If they have no frame of reference then it seems like their general contentedness is being measured (or as another commenter indicated, their willingness to gripe.)
Yes.
Even in your example, you propose an individual could know multiple kinds of after-sleep feelings.
I believe that how you feel after a night's sleep is mostly due to nutrition (long term, not just the day before).
that might be true unless we can define what nutrition can help with sleep. There's plenty of people who eat well and sleep like crap.
If you're eating "well" but don't feel/sleep well, are you really eating well?