The basic idea is that every 4 hours of xp'ing, your rest state drops to the next tier. Your XP gain is multiplied by your rest state, so well rested means you get double XP, exhausted (Would be in the 20th straight hour of xping... Noreen's going to be upset) would give you 25% xp.
You get back your rest state by logging out, at an Inn you recover up to Well rested (200% xp) and anyplace else you get Normal (100%).
Generally speaking I think I like the idea, seems like a mechanism to slow the advance of the 14-hour a day players. Not sure it's perfect.
btw, im selling uber WoW RV's on ebay...take your mobil home anywhere!
I'm betting the hardcore folks will eithert suck it up and take the xp penalty, or use the time to level up an alt.