Stable for me is one when I sleep a decent amount of hours but not too many eiher.
When I wake up and I don't either:
1. Really dread the day and will not get out bed no matter what,
OR
2. I don't have a gad-zillion things I have to/want to get done right NOW!, all of them this instant,
so then the day actually lies somewhere in between; this means it is as stable and levelish as it is going to get.