Telling her that he literally "sleeps like he's dead" is probably not the best thing to say, even if it's true. He also had to be able to fall asleep, first, to sleep that "soundly" and that's a completely different struggle. The TARDIS will be fine--that's all he has to keep telling himself.

"Can't say I've ever slept next to myself, so I don't know." He grins and flops down next to her, burrowing under sheets since they're not infested with fleas like the last time. "Just don't kick me off the bed and we'll be good."