That's because the something (or rather, someone) that *had* gone to sleep there had returned to being human-shaped at some point during the night. He'd pretty much been asleep at the time, so the idea of waking up Xander to go "oh, hey, so I'm me-shaped again" hadn't occurred to him at all. Instead, he'd just stretched out on the bed and rolled over to a more typical asleep-with-the-boyfriend position.