I have more dreams to remember if I oversleep (sleep more than 10 hrs), however the early ones are fairly unclear. But oversleeping increases my chances of lucidity. My dream recall is next to nothing if I undersleep, which would be like sleeping under 6 hours. So I'd agree, it's got an optimal amount which is just being well rested. I think I might have to start using a slightly different sleeping schedule because my current one is doing me no good.