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.
Whenever you sleep significantly less or significantly more than the average time you sleep every day, you will feel sluggish.
Unless you sleep something like an hour, then sleep deprivation euphoria kicks in and you feel hyper all day, but can't actually think at all.