The Surprising Relationship Between Sleep and Learning

edit: yes.. was a bit sleepy..

Correlation, not causation.

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.

Yes. But it doesn't relate to actual hours of sleep, but to what you're used to.

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.

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.