On the other hand, maybe its just me (which I don't think), but has anyone else noticed that when you get >8.5h of sleep, you're actually less productive the next day? I always feel sluggish the whole day when I get that much sleep.
One possible root cause for >8.5 hours sleep is that you have heavy sleep debt and need the sleep, in which case one night isn't enough to fix the whole debt, and so of course you'll still be tired.
Another possible root cause is that you might have sleep apnea, and on that particular night you were not breathing very much and so your sleep was not restful.
I often sleep more than 8.5hrs and I think there might be more here than meets the eye. For one thing I think if you sleep longer you're more likely to wake up dehydrated which makes you feel the sleep didn't do you well. You can avoid this by chugging water when you wake or by drinking water during the night. Also I think folks with allergies can feel worse if they sleep longer, if they're breathing more dust from pillows etc.