There's a tiredness sleep fixes and a tiredness it doesn't touch. Nine hours and you wake flat: that's the second kind, and it isn't a sign that something is wrong with you.
Rest seems to work in layers. The body goes first, in a night or two. The rest of it takes longer and returns unevenly — an hour here, one decent morning there.
So a new day may arrive with a great deal more in it, or with only slightly more. Both are the process working. The mornings improve in the aggregate, even when a particular one doesn't.