I'm not sure length really has any factor on how well you sleep. I've had two hours of sleep before and felt great (read: UFO demo), and then have 10 hours of sleep and feel more tired than when I went to sleep.
I have this theory that you only get a really good night's sleep when you have one of those long dreams that you actually remember some of after you wake up.
But in my dream, you and I were killing zombies.
So that can't be it, either!
Whut. Then I have no idea. Sleep is a mysterious thing...
We died at the end, so that may be a part of it.
:(
:( Maybe. When I was scum in mafia, I kept dreaming about the game over and over (since it was sadly the main focus of my Spring Break) about how I was going to screw up, and I woke up feeling groggy every morning.