Sleeping is useless but rested exp as in many mmorpgs is nice thing. For example per 1 hour of inactivity you get 5% of rested exp that equal your lvl. So on X lvl if you dont play for 10 hours you will get 100% bonus exp from all sources for the amount of 50% exp needed to lvl up.
Maximum of 10 hours for 100, and having it last for a few hours, and I'd be down. Or cut that in half to just 50, and Loner and Swift Leaner could jump that up to 70 (120 if we did go with the ten hours.) Either case, it'd still be pretty cool.
This may need to make it real time hours of waiting and potentially end on death as well as time. I mean, if you're getting close to double XP, there probably needs form of frustration that needs to exist. Dying after waiting a quarter of a day would probably do it. Perhaps it can only work with XP gained through combat, not crafting.
Would be nice to give those items in our bases, tens and houses more purpose than decoration.
Still, I can do without.