The Joker wasn't exactly one to remember every little detail about his day-to-day life. That was too much effort, especially when you have the habit of completely ignoring events in favor for the "bigger picture." Still, he was pretty sure that when he'd finally fallen asleep (for the first time in he didn't know how long), things had been a little different than they were now.
For one, he'd been in the other room and for two he hadn't been anywhere near Harvey. (He was almost certain that he'd taken off his shirt and shoes himself, though, so that was a bonus.)
So, waking up when his foot kicked up against the-suddenly-next-to-him-Harvey's leg, he was suitably baffled. Not that it really mattered. It actually mattered so little that he just sort of... threw his arm around Harvey's chest and worked on going back to sleep.
Hey, he was tired, what can you do?