It's the exact same for me. It's like at eight hours exactly something "clicks" and you're fresh, but anything before that is a weird uncompleted process where I feel like garbage all day yet I can't go back to sleep. I can even feel the weight of my eye bags. Then, I take a short (20 min) nap at some point in the day and I'm completely recovered.

It's frustrating because just 15 more minutes would have prevented all that suffering.