So I've figured out what makes sleeping so restful to myself at least. There are probably a myriad of small subconscious things, like restoring levels of some neuro-chemical or repairing tissue damage. But those things aren't what make sleep important to me. No, for me sleep makes a division between my days and activities.
I receive so much information and do some many things that without some clear divisions in my memory, everything runs together. Which prevents me from feeling refreshed. That's why I think I find the school week so long and tiring, it's because I don't get enough sleep and it just seems like a continuous day to me.I need a long period of inactivity to partition my memories.
Also, I am a lazy bum who enjoys sleeping for hours on end.