My little guy used to sleeping in his litter boxwhen he was in his cage, too. Possibly because he didn't like the metalbars of his cage floor! He'd go inside to sleep at night (where he usedto have a comfortable bed), but if he was ever in his cage during theday he'd sleep in his litter box.
I didn't know that bunnies don't like metal bars!! Well, next bunny we get will NOT have that old cage...
My little guy mostly just slept in his cage at night and was insidemost of the day though...and I never caught him sleeping in the insidelitter tray (and who'd want to...the cats used it too!!)
Fiona